arch: riscv: Use CONFIG_PMP_SLOTS for M-mode stack guard arrays

The arrays `m_mode_pmpaddr_regs` and `m_mode_pmpcfg_regs` within the
`_thread_arch` struct, used for the M-mode stack guard (when
CONFIG_PMP_STACK_GUARD is enabled), were previously sized using the
hardcoded PMP_M_MODE_SLOTS macro (defined as 8).

This commit changes the array sizing to use the Kconfig option
CONFIG_PMP_SLOTS. This option reflects the total number of Physical
Memory Protection (PMP) slots available and configured for the target
hardware.

Using CONFIG_PMP_SLOTS ensures these arrays are dimensioned according
 to the system's actual capabilities, providing better flexibility and
correctness over a fixed size.

Signed-off-by: Firas Sammoura <[email protected]>

Author: fsammoura1980

include/zephyr/arch/riscv/thread.h

@@ -81,8 +81,8 @@ struct _thread_arch {
 #endif
 #ifdef CONFIG_PMP_STACK_GUARD
 	unsigned int m_mode_pmp_end_index;
-	unsigned long m_mode_pmpaddr_regs[PMP_M_MODE_SLOTS];
-	unsigned long m_mode_pmpcfg_regs[PMP_M_MODE_SLOTS / sizeof(unsigned long)];
+	unsigned long m_mode_pmpaddr_regs[CONFIG_PMP_SLOTS];
+	unsigned long m_mode_pmpcfg_regs[CONFIG_PMP_SLOTS / sizeof(unsigned long)];
 #endif
 };