powerpc/83xx: Fix the interrupt loss problem on ipic

The interrupt pending register is write 1 clear.  If there are more than
one external interrupts pending at the same time, acking the first
interrupt by reading pending register then OR the corresponding bit and
write back to pending register will also clear other interrupt pending
bits.  That will cause loss of interrupt.

Signed-off-by: Da Yu <dayu@datangmobile.cn>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>