doc: st: stm32mp1: Add FIP support for trusted boot

TF-A for STM32MP15 now supports the FIP: it is a packaging format which
includes the secure monitor, u-boot-nodtb.bin and u-boot.dtb

This FIP file is loaded by FSBL = TF-A BL2.

This patch updates the board documentation to use this FIP file and no
more u-boot.stm32 (with STM32 image header) which is no more generated.
Signed-off-by: NPatrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: NPatrice Chotard <patrice.chotard@foss.st.com>