xen/arm: gic-v3: Bail out if gicv3_cpu_init fail

xen/arm: gic-v3: Bail out if gicv3_cpu_init fail

When system registers are not enabled, all the access to them will trap
in EL2. In Xen, system registers will be enabled by gicv3_cpu_init only
on success. As the rest of the code (e.g gicv3_hyp_init) relies on
system register, it is better to bail out directly.

This will save time on debugging early boot issue on GICv3 platform.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>