[Patch ARM] Support MVFR2 VFP Coprocessor register for ARMv8-A

This patch adds support mvfr2 control registers for armv8-a as
this was missed from the original port to armv8-a (documented
at G6.2.109 in (Issue B.a) of the ARM-ARM. This was discovered
by an internal user of the GNU toolchain.

I'd like to backport this to the binutils 2.28 and binutils 2.29
release branch if possible (with suitable testing and basically
checking removing the armv8-r parts).

regards Ramana

2017-07-04  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

        * gas/config/tc-arm.c (arm_regs): Add MVFR2.
        (do_vmrs): Constraint for MVFR2 and armv8.
        (do_vmsr): Likewise.
        * gas/testsuite/gas/arm/armv8-a+fp.d: Update.
        * gas/testsuite/gas/arm/armv8-a+fp.s: Likewise.
        * gas/testsuite/gas/arm/vfp-bad.s: Likewise.
        * gas/testsuite/gas/arm/vfp-bad.l: Likewise.
        * opcodes/arm-dis.c: Support MVFR2 in disassembly
        with vmrs and vmsr.