[MIPS] Fix broken rm7000/rm9000 interrupt handling

Properly acknowledge RM7K and RM9K interrupts. Before this, interrupts were
permanently masked after their first occurrence, making them non-functional.

Signed-off-by: Thomas Koeller <thomas.koeller@baslerweb.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>