powerpc: helper functions to initialize AMR, IAMR and UAMOR registers

Introduce  helper functions that can initialize the bits in the AMR,
IAMR and UAMOR register; the bits that correspond to the given pkey.

Reviewed-by: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>