MIPS: Alchemy: Only build AU1000 INTC code for compatible cpus

Use the GPIO config symbol to only build Au1000 interrupt code on chips with
compatible hw.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/670/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>