stm32mp1: remove the second TAMP_BOOT_CONTEXT update

The register TAMP_BOOT_CONTEXT is already updated in
get_bootmode() in cpu.c and no need to be done
twice.

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