MAINTAINERS: Add arm64 selftests to the ARM64 PORT entry

Since these are tests specific to the arm64 architecture, it makes sense
for the arm64 maintainers to gatekeep the corresponding changes.

Cc: Shuah Khan <shuah@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>