KVM: arm64: Inherit RESx bits from FGT register descriptors

KVM: arm64: Inherit RESx bits from FGT register descriptors

The FGT registers have their computed RESx bits stashed in specific
descriptors, which we can easily use when computing the masks used
for the guest.

This removes a bit of boilerplate code.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Link: https://patch.msgid.link/20260202184329.2724080-7-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>