arm64: KVM: system register handling

Provide 64bit system register handling, modeled after the cp15
handling for ARM.

Reviewed-by: Christopher Covington <cov@codeaurora.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Git-commit: 7c8c5e6a9101ea57a1c2c9faff0917e79251a21e
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[imaund@codeaurora.org: resolved merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>