powerpc: Handle exceptions caused by pkey violation

Handle Data and  Instruction exceptions caused by memory
protection-key.

The CPU will detect the key fault if the HPTE is already
programmed with the key.

However if the HPTE is not  hashed, a key fault will not
be detected by the hardware. The software will detect
pkey violation in such a case.

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