KVM: arm64: gic-v5: Add GICv5 EL1 sysreg userspace accessors

Now that KVM is able to run meaningful VMs with GICv5, it is important
to be able to save and restore the GICv5 state in order to support VM
migration.

Add functions to handle userspace reads and writes of the GICv5 EL1
system registers. Some architected register values are reconstructed
from KVM's internal representation rather than stored as raw register
state. For example, ICC_CR0_EL1.En and ICC_PCR_EL1.Priority are backed
by fields in the saved virtual VMCR, while the PPI Enable, Active, and
Pending registers are assembled from per-interrupt vgic_irq state.
Userspace writes are translated back into the corresponding internal
representation.

Wire the accessors into KVM_DEV_ARM_VGIC_GRP_CPU_SYSREGS for the GICv5
device, reusing the existing GICv3 CPU sysreg UAPI encoding. Require
the VGIC to be initialised before accessing the register state.

Allow userspace to read the state after a vCPU has run, but reject
writes once any vCPU has run. This ensures that userspace restores the
GICv5 CPU interface state before KVM finalises it for guest execution.

Signed-off-by: Sascha Bischoff <sascha.bischoff@arm.com>
Link: https://patch.msgid.link/20260904113404.4051341-32-sascha.bischoff@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

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