KVM: arm64: Add AT fast-path support for S1PIE

KVM: arm64: Add AT fast-path support for S1PIE

Emulating AT using AT instructions requires that the live state
matches the translation regime the AT instruction targets.

If targeting the EL1&0 translation regime and that S1PIE is
supported, we also need to restore that state (covering TCR2_EL1,
PIR_EL1, and PIRE0_EL1).

Add the required system register switcheroo.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20241023145345.1613824-19-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>