selftests/powerpc: Skip 4PB test on 4K PAGE_SIZE systems

Systems using the hash MMU with a 4K page size don't support 4PB address
space, so skip the test because the bug it tests for can't be triggered.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220901020215.254097-1-mpe@ellerman.id.au