openrisc: include l.swa in check for write data pagefault

During page fault handling we check the last instruction to understand
if the fault was for a read or for a write.  By default we fall back to
read.  New instructions were added to the openrisc 1.1 spec for an
atomic load/store pair (l.lwa/l.swa).

This patch adds the opcode for l.swa (0x33) allowing it to be treated as
a write operation.

Signed-off-by: Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>
[shorne@gmail.com: expanded a bit on the comment]
Signed-off-by: Stafford Horne <shorne@gmail.com>