arm64/sve: Generalise vector length configuration prctl() for SME

In preparation for adding SME support update the bulk of the implementation
for the vector length configuration prctl() calls to be independent of
vector type.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211210184133.320748-3-broonie@kernel.org


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