selftests/powerpc: Move Hash MMU check to utilities

This moves a function to test if the MMU is in Hash mode
under the generic test utilities.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200604125610.649668-3-sandipan@linux.ibm.com