doc: stm32mp1: Use u-boot.itb if CONFIG_SPL_LOAD_FIT=y

For systems where SPL loads fitImage, i.e. CONFIG_SPL_LOAD_FIT=y, use
u-boot.itb in the relevant documentation parts. Otherwise use u-boot.img.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>