arm64: Document missing userspace visible fields in ID_AA64ISAR2_EL1

We have exposed a number of fields in ID_AA64ISAR2_EL1 to userspace without
adding the matching documentation in cpu-feature-registers.rst, update it
to match the implementation.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20230914-arm64-feat-hbc-doc-v1-2-797d25f06897@kernel.org


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