[AArch64] Add ARMv8.2-A persistent memory instruction

ARMv8.2-A adds the "dc cvap" instruction, which is a system instruction
that cleans caches to the point of persistence (for systems that have
persistent memory). It is a required part of ARMv8.2-A, so no additional
subtarget features are required.

Differential Revision: http://reviews.llvm.org/D15016

llvm-svn: 254156