arm64: Add encoding for POR_EL2

POR_EL2 is the equivalent of POR_EL1 for the EL2&0 translation
regime, and it is sorely missing from the sysreg file.

Add the sucker.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241023145345.1613824-28-maz@kernel.org


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>