KVM: arm64: Document registers exposed via KVM_DEV_ARM_VGIC_GRP_CPU_SYSREGS

We never documented which GICv3 registers are available for save/restore
via the KVM_DEV_ARM_VGIC_GRP_CPU_SYSREGS interface.

Let's take the opportunity of adding the EL2 registers to document the whole
thing in one go.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250714122634.3334816-12-maz@kernel.org


[ oliver: fix trailing whitespace ]
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>