kselftest/arm64: Add test coverage for ZT register signal frames

We should have a ZT register frame with an expected size when ZA is enabled
and have no ZT frame when ZA is disabled. Since we don't load any data into
ZT we expect the data to all be zeros since the architecture guarantees it
will be set to 0 as ZA is enabled.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-sme2-v4-18-f2fa0aef982f@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>