[AArch64] Add ARMv8.2-A ID_A64MMFR2_EL1 register

ARMv8.2-A adds a new ID register, ID_A64MMFR2_EL1, which behaves in the
same way as ID_A64MMFR0_EL1 and ID_A64MMFR1_EL1. It is a required part
of ARMv8.2-A, so no additional subtarget features are required.

Differential Revision: http://reviews.llvm.org/D15017

llvm-svn: 254155