selftests/mm: va_high_addr_switch should skip unsupported arm64 configs

va_high_addr_switch has a mechanism to determine if the tests should be
run or skipped (supported_arch()). This currently returns
unconditionally true for arm64. However, va_high_addr_switch also
requires a large virtual address space for the tests to run, otherwise
they spuriously fail.

Since arm64 can only support VA > 48 bits when the page size is 64K,
let's decide whether we should skip the test suite based on the page
size. This reduces noise when running on 4K and 16K kernels.

Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>