powerpc/8xxx: Fix interrupt handling in MPC8xxx GPIO driver

Interrupt handler in MPC8xxx GPIO driver is missing the call to PIC EOI
(end of interrupt) handler. As a result, at least on 85XX systems, GPIO
interrupt is delivered only once. This patch adds the missing EOI call.
Tested on custom P1022 board.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>