KVM: arm64: Enforce absence of FEAT_FGT on FGT registers

KVM: arm64: Enforce absence of FEAT_FGT on FGT registers

As we want to enforce FGT registers behaving as RES0 when FEAT_FGT
is not exposed to the guest, We move a bumch of things that are
so far passed as parameter into a structure that points to the
bit description.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>