drivers: serial: stm32: Refactor for PM handling

[knthm]

Summary

This PR refactors the initialization/configuration logic of STM32 UART driver. It is intended to complement PR #59200 by making reinitialization of the peripheral clocks and registers more straightforward, for when the SoC loses their context and contents in certain low-power modes.

In refactoring I've also stumbled on a few bugs that I've fixed.

• Added static uart_config struct to STM32_UART_INIT macro
• Added uart_config struct pointer as uart_stm32_config member
• (Re)moved now superfluous baud_rate, parity, and hw_flow_control struct members
• Moved overlapping baudrate/parity/data-/stop-bits/flow control configuration to common function, with persistence in static instance uart_config struct
• Removed incorrect runtime parity/data bits configuration, and replaced with thoroughly tested compile-time BUILD_ASSERT statements
  • Original runtime logic in init forced a very small subset of UART configurations, and straight-up ignored many valid DT property combinations (e.g 7E1)
  • This now also allows invalid configurations to get flagged compile-time
  • The previous logic partially conflicted with the logic in uart_stm32_configure, especially that relating to parity/data-bits combinations, where either enforced opposite configurations
• The #if defined(LL_USART_STOPBITS_1_5) && HAS_LPUART_1 conditional broke "half-bit" parity application configurations of USARTs, this is now fixed

Change Description

Add static uart_config struct to init macro with corresponding pointer as uart_stm32_config struct member. This struct will store boot and runtime UART configuration for reinitialization upon exiting low-power modes, where register contents might be lost.

Remove hw_flow_control, parity, and baud_rate from uart_stm32_config and uart_stm32_data structs, respectively, as the need for these is now obviated by the presence of the uart_config struct, which contains equivalent members.

Add default baudrate, parity, stop, and data bits constants, used to initialize the uart_config struct, in case the corresponding UART DT properties are not set.

Move overlapping UART parameter configuration to new uart_stm32_parameters_set function. This function is called by uart_stm32_configure upon application/subsystem configuration, and uart_stm32_registers_configure upon (re-)initialization of driver instances.

Fix flawed stop bit/LPUART conditionals, which didn't allow for 0.5/1.5 stop bit USART configurations via application uart_configure syscalls.

Replace incorrect and limited runtime parity/data bits conditional configuration with build-time logic using BUILD_ASSERT. This allows for more flexible UART configurations, while preventing invalid DTS configurations at build-time.

Issues Closed by this PR

• 7-bit data length with (even/odd) parity is configured as 8-bit in UART #56003

Related Issues or Pull Requests

• stm32: Add reinitialization feature for SPI and UART #59200

[knthm]

• Split into individual commits to assist review

[erwango]

Looks ok generally.
The only point is the use of new struct in the device const config struct while in theory runtime updatable parameters should go in data struct.

[knthm]

• Add separate commit for include directives move
• Add separate commit for moving reset configuration to config struct
• Move uart_config pointer to uart_stm32_data struct
• Fix indentation and remove inline keyword from function called more than once
• Simplify DTS stop bit BUILD_ASSERT check

[knthm]

• Rebase on main and resolve conflicts

[gautierg-st]

Some comments about the datawidth check, otherwise looks good

[erwango]

#54354 has been merged. Please rebase

[knthm]

• Rebase on main and resolve conflicts