kselftest/arm64: Enumerate SME2 in the signal test utility code

Support test cases for SME2 by adding it to the set of features that we
enumerate so test cases can check for it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-16-f2fa0aef982f@kernel.org


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