boards: st: add support for stm32mp257f_dk board

Introduce initial board support for the STM32MP257F-DK Discovery Kit.
This includes:
- Board metadata (board.yml, stm32mp257f_dk.yaml)
- Base software configuration (Kconfig.stm32mp257f_dk)
- CMake build integration (board.cmake)
- Default configuration (stm32mp257f_dk_stm32mp257fxx_m33_defconfig)
- Minimal device tree (stm32mp257f_dk_stm32mp257fxx_m33.dts)
- Initial documentation and image for the stm32mp257f_dk board.
- OpenOCD configuration files for debugging support.

Signed-off-by: Luc BEAUFILS <luc.beaufils@savoirfairelinux.com>