MIPS: BCM63XX: Handle 64 bits irq stat register in irq code.

bcm6368 has larger irq registers, prepare for this.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2898/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>