soc: stm32: add initial soc support for stm32mp2x

Add initial soc support for the stm32mp2x series, including
initial Kconfig entries and default configuration files.
This enables Zephyr to recognize and build for the stm32mp2x series,
taking the stm32mp257f_ev1 as a baseline.

Includes:
- Kconfig and defconfig files for SoC selection and defaults
- soc.h for hal headers
- CMakeLists.txt for build system integration
- soc.yml update to register the new SoC

System Clock is configured statically from DTS. So no initialization
hook or soc.c needed.

Signed-off-by: Youssef Zini <youssef.zini@savoirfairelinux.com>