arch-arm: Add FP16 support introduced by Armv8.2-A

arch-arm: Add FP16 support introduced by Armv8.2-A

This changeset adds support for FP/SIMD instructions with
half-precision floating-point operands.

Change-Id: I4957f111c9c5e5d6a3747fe9d169d394d642fee8
Signed-off-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/13084
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>