KVM: arm64: Use FGT feature maps to drive RES0 bits

Another benefit of mapping bits to features is that it becomes trivial
to define which bits should be handled as RES0.

Let's apply this principle to the guest's view of the FGT registers.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>