doc: st: stm32mp1: add STM32MP13x support

Add in U-Boot documentation the quick instruction to
setup the STMicroelectronics STM32MP13x boards.

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