arm64: Document boot requirements for FEAT_SME_FA64

The EAC1 release of the SME specification adds the FA64 feature which
requires enablement at higher ELs before lower ELs can use it. Document
what we require from higher ELs in our boot requirements.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211026111802.12853-1-broonie@kernel.org


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