arm64/hbc: Document HWCAP2_HBC

When we added support for FEAT_HBC we added a new hwcap but did not
document that we had done so, add the documentation.

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-v1-1-797d25f06897@kernel.org


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