armv8: layerscape: Make U-Boot EL2 safe

When U-Boot boots from EL2, skip some lowlevel init code requiring
EL3, including CCI-400/CCN-504, trust zone, GIC, etc. These
initialization tasks are carried out before U-Boot runs. This applies
to the RAM version image used for SPL boot if PPA is loaded first.

Signed-off-by: York Sun <york.sun@nxp.com>