arm64/sve: Add stub for sve_max_virtualisable_vl()

Fixes build problems for configurations with KVM enabled but SVE disabled.

Reported-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211022141635.2360415-2-broonie@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>