arm64/sme: Add basic enumeration for SME2

Add basic feature detection for SME2, detecting that the feature is present
and disabling traps for ZT0.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-8-f2fa0aef982f@kernel.org


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