target-ppc: implement load atomic instruction

lwat: Load Word Atomic
ldat: Load 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>
[ combine both lwat/ldat implementation using macro ]
Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>