ANDROID: KVM: arm64: Add atomics-based checking refcount implementation at EL2

The current nVHE refcount implementation at EL2 uses a simple spinlock
to serialise accesses. Although this works, it forces serialisation in
places where it is not necessary, so introduce a simple atomics-based
refcount implementation instead.

Signed-off-by: Will Deacon <will@kernel.org>
Bug: 278749606
Change-Id: I238861fe1cc2880c552c239cdd707dfa8a400be2