target-ppc: implement store atomic instruction

stwat: Store Word Atomic
stdat: Store Doubleword Atomic

The instruction includes as function code (5 bits) which gives a detail
on the operation to be performed. The patch implements five such
functions.

Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
Signed-off-by: Harish S <harisrir@linux.vnet.ibm.com>
Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
[ implement stdat, use macro and combine both implementation ]
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>