MIPS: Alchemy: merge Au1000 and Au1300-style IRQ controller code.

With a generic plat_irq_dispatch (for Alchemy at least) code for both
interrupt controller types can coexist in a single kernel image and be
autodetected at runtime.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2935/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>