arm64: KVM: Turn system register numbers to an enum

Having the system register numbers as #defines has been a pain
since day one, as the ordering is pretty fragile, and moving
things around leads to renumbering and epic conflict resolutions.

Now that we're mostly acessing the sysreg file in C, an enum is
a much better type to use, and we can clean things up a bit.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@linaro.org>