KVM: arm64: vgic-v3: Drop userspace write sanitization for ID_AA64PFR0.GIC on GICv5

KVM: arm64: vgic-v3: Drop userspace write sanitization for ID_AA64PFR0.GIC on GICv5

Drop a check that blocked userspace writes to ID_AA64PFR0_EL1 for
writes that set the GIC field to 0 (NI) on GICv5 hosts. There is no
such check for GICv3 native systems, and having inconsistent behaviour
both complicates the logic and risks breaking existing userspace
software that expects to be able to write the register.

This means that userspace is now able to create a GICv3 guest on GICv5
hosts, and disable the guest from seeing that it has a GICv3. This
matches the already existing behaviour for GICv3-native VMs, allowing
for fewer issues when migrating from GICv3 hosts to compatible GICv5
hosts.

Additionally, this allows the trap and FGU infrastucture to kick in as
these rely on the state of the feature bits that have been set.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260319154937.3619520-2-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>