selftests/arm64: Add MTE test config fragment

The arm64 selftest collection has no Kconfig fragment, so kernels built
with the selftest requirements are not guaranteed to provide the support
used by these tests.

Add a fragment covering all tests in arm64.

Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>