drivers/gicv3: use mpidr to probe GICR for current CPU
drivers/gicv3: use mpidr to probe GICR for current CPU

In function gicv3_rdistif_probe(), line #1322 implies
gicv3_driver_data->mpidr_to_core_pos() may be null, but the original
code uses this interface to get current CPU index unconditionally.

It is better to use MPIDR to probe GICR which does not depend on
gicv3_driver_data->mpidr_to_core_pos().

Signed-off-by: Heyi Guo <554902d6d74fca466aa06b42f6ec0849d7c72a90@linux.alibaba.com>
Change-Id: I64add055385040fe0a56b977e2299608e2309a6e

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware