arm64/sme: Document ABI for TPIDR2 signal information

In order to allow access to TPIDR2 from signal handlers we need to add it
to the signal context, document that we are doing so.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-tpidr2-sig-v3-1-c77c6c8775f4@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>