armv8: layerscape: don't initialize GIC in SPL

The BL31 expects the GIC to be uninitialized. Thus, if we are loading
the BL31 by the SPL we must not initialize it. If u-boot is loaded by
the SPL directly, it will initialize the GIC again (in the same
lowlevel_init()).

This was tested on a custom board with SPL loading the BL31 and jumping
to u-boot as BL33 as well as loading u-boot directly by the SPL. In case
the ATF BL1/BL2 is used, this patch won't change anything, because no
SPL is used at all.

Signed-off-by: Michael Walle <michael@walle.cc>