board: st: stm32mp2: add checkboard

Implement the weak function checkboard to identify the used board with
compatible in device tree for the support of stm32mp2 STMicroelectronics
boards.

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