arm64/idreg: Don't disable SME when disabling SVE

SVE and SME are separate features which can be implemented without each
other but currently if the user specifies arm64.nosve then we disable SME
as well as SVE. There is already a separate override for SME so remove the
implicit disablement from the SVE override.

One usecase for this would be testing SME only support on a system which
implements both SVE and SME.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230315-arm64-override-sve-sme-v2-1-bab7593e842b@kernel.org


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