MIPS: ath79: Add OF support to the IRQ controllers

Add OF support for the CPU and MISC interrupt controllers of most
supported ATH79 devices.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>