KVM: arm64: Don't advertise ICH_*_EL2 registers through GET_ONE_REG

KVM: arm64: Don't advertise ICH_*_EL2 registers through GET_ONE_REG

It appears that exposing the GICv3 EL2 registers through the usual
sysreg interface is not consistent with the way we expose the EL1
registers. The latter are exposed via the GICv3 device interface
instead, and there is no reason why the EL2 registers should get
a different treatement.

Hide the registers from userspace until the GICv3 code grows the
required infrastructure.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250714122634.3334816-3-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>