arm64: Add CNTVCT_EL0 trap handler

Since people seem to make a point in breaking the userspace visible
counter, we have no choice but to trap the access. Add the required
handler.

Change-Id: I6181a57dad00442fc1f4dfa2729e911d539d0736
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Git-commit: 6126ce0588eb5a0752d5c8b5796a7fca324fd887
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[kyan@codeaurora.org: resolve trivial merge conflicts]
Signed-off-by: Kyle Yan <kyan@codeaurora.org>