arm64/sysreg: Allow a 'Mapping' descriptor for system registers

*EL02 and *_EL12 system registers are actually only accessors for
EL0 and EL1 registers accessed from EL2 when HCR_EL2.E2H==1. They
do not have fields of their own.

To that effect, introduce a 'Mapping' entry, describing which
system register an _EL12 register maps to.

Implementation wise, this is handled the same was as Fields,
which ls only a comment.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241219173351.1123087-2-maz@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>