KVM: arm64: Validate FGT register descriptions against RES0 masks

In order to point out to the unsuspecting KVM hacker that they
are missing something somewhere, validate that the known FGT bits
do not intersect with the corresponding RES0 mask, as computed at
boot time.

THis check is also performed at boot time, ensuring that there is
no runtime overhead.

Signed-off-by: Marc Zyngier <maz@kernel.org>