arm64: Document boot requirements for SME 2

SME 2 introduces the new ZT0 register, we require that access to this
reigster is not trapped when we identify that the feature is supported.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-2-f2fa0aef982f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>