stm32: stm32u0 clock dt-binding: the RTC_SEL macro should use the BDCR_REG value

[ExaltZephyr]

Describe the bug

In stm32u0_clock.h:90
The RTC_SEL macro should use the BDCR_REG register value instead of CSR_REG, the register CSR_REG does not exist for the STM32U0 boards.

To Reproduce
The issue can be seen when using the RTC_SEL macro on the devicetree, as of now there's no support for the RTC, but it will be visible once an RTC device is supported.

Expected behavior
It is expected for the macro RTC_SEL to use BDCR_REG register so the dts build would not fail.

Impact
Minimal impact, however, it blocks future support of the RTC device.

Environment (please complete the following information):
OS: Windows
Toolchain: Zephyr SDK
Commit SHA or Version used: ed48de2

Additional context

- #define RTC_SEL(val)		STM32_DOMAIN_CLOCK(val, 3, 8, CSR_REG)
+ #define RTC_SEL(val)		STM32_DOMAIN_CLOCK(val, 3, 8, BDCR_REG)

[henrikbrixandersen]

Please use our bug template when reporting bugs. You need to edit this issue to include the information requested in https://github.com/zephyrproject-rtos/zephyr/blob/main/.github/ISSUE_TEMPLATE/001_bug_report.md

[erwango]

Thanks for raising this point. Don't hesitate to submit a PR to fix it.

[rruuaanng]

If we can, I'll commit a PR :)