board: st: stmp32mp1: Use BUTTON UCLASS in board_key_check()

Instead of using gpio directly to detect key pressed on button
dedicated for fastboot and stm32mprog, make usage of BUTTON UCLASS.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
1 file changed