arm64/sme: Provide ABI documentation for SME

Provide ABI documentation for SME similar to that for SVE. Due to the very
large overlap around streaming SVE mode in both implementation and
interfaces documentation for streaming mode SVE is added to the SVE
document rather than the SME one.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Link: https://lore.kernel.org/r/20220419112247.711548-5-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>