arm, dev: add basic support for GICC_BPR register

arm, dev: add basic support for GICC_BPR register

The Binary Point Register (BPR) specifies which bits belong to the
group priority field (which are used for preemption) and which to the
subpriority field (which are ignored for preemption).

Change-Id: If51e669d23b49047b69b82ab363dd01a936cc93b
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/2443
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Weiping Liao <weipingliao@google.com>