For I/O access, 810/807 store instruction fast retire will cause wrong
primitive. For example:

	stw (clear interrupt source)
	stw (unmask interrupt controller)
	enable interrupt

stw is fast retire instruction. When PC is run at enable interrupt
stage, the clear interrupt source hasn't finished. It will cause another
wrong irq-enter.

So use mb() to prevent above.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Lu Baoquan <lu.baoquan@intellif.com>