stm32mp1: move stboard command in board/st/common directory

Move the ST command in board/st/common, as this command is only used
by ST board. Prepare the support in U-Boot of boards with STM32MP15x
SOC but not STMicroelectronics.

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