boards: add gpio, button and led support

Add gpio support for STM32MP257F-EV1 board.
Add the orange (&led0) LED to the device tree and enable GPIOJ.
Add the user button USER2 to the device tree and enable GPIOG.
This step allows building and debugging the blinky and button sample
applications.

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