arm64/sve: Add some comments for sve_save/load_state()

The use of macros for the actual function bodies means legibility is always
going to be a bit of a challenge, especially while we can't rely on SVE
support in the toolchain, but this helps a little.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210812201143.35578-1-broonie@kernel.org


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