MIPS: Alchemy: irq: use runtime CPU type detection

Use runtime CPU detection instead of relying on preprocessor symbols.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/701/


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