arm64/sve: Document that __SVE_VQ_MAX is much larger than needed

arm64/sve: Document that __SVE_VQ_MAX is much larger than needed

__SVE_VQ_MAX is defined without comment as 512 but the actual
architectural maximum is 16, a substantial difference which might not
be obvious to readers especially given the several different units used
for specifying vector sizes in various contexts and the fact that it's
often used via macros.  In an effort to minimise surprises for users who
might assume the value is the architectural maximum and use it to do
things like size allocations add a comment noting the difference, and
add a note for SVE_VQ_MAX to aid discoverability.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Dave Martin <Dave.Martin@arm.com>
Link: https://lore.kernel.org/r/20240209-arm64-sve-vl-max-comment-v2-1-111b283469ee@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>