Add test for AArch64 cache instructions

[Ivan-Velickovic]

This patch adds a simple test to ensure that the instructions for cache maintenance from user-space succeed (when the kernel is configured to allow it).

It does not test that the cache operations produce the right side affects, just that they don't cause a fault/exception.

I did not add a test to check that a fault does occur if CONFIG_AARCH64_USER_CACHE_ENABLE is not enabled, since sel4test is only run with that config operation enabled.