powerpc/mm/radix: Setup AMOR in HV mode to allow key 0

Setup AMOR (Authority Mask Override Register) in HV mode so that the
host and guest kernel can in turn setup IAMR.

This allows us to enable key 0 in a following patch.

Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>