[AArch64] Adding support for PMMIR_EL1 register

Summary:
The PMMIR_EL1 register is present in Armv8.4 with PMU extension.
This patch adds support for it.

Reviewers: t.p.northover, dnsampaio

Reviewed By: dnsampaio

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68940

llvm-svn: 375228