Contrary to what was originally expected, EL3 firmware can (for whatever
reason) disable GICv3 system register access. In this case, the kernel
explodes very early.

Work around this by testing if the SRE bit sticks or not. If it doesn't,
abort the GICv3 setup, and pray that the firmware has passed a DT that
doesn't contain a GICv3 node.
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>