arm64/sve: Add a comment documenting the binutils needed for SVE asm

At some point it would be nice to avoid the need to manually encode SVE
instructions, add a note of the binutils version required to save looking
it up.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20210816125024.8112-1-broonie@kernel.org


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