MIPS: BCM63XX: Prepare irq code to handle different external irq hardware implementation.

External irq only works for 6348, change code to prepare support of
other CPUs.

Signed-off-by: Maxime Bizon <mbizon@freebox.fr>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2895/


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