irq_domain/mips: Allow irq_domain on MIPS

This patch makes IRQ_DOMAIN usable on MIPS.  It uses an ugly workaround
to preserve current behaviour so that MIPS has time to add irq_domain
registration to the irq controller drivers.  The workaround will be
removed in Linux v3.6

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mips@linux-mips.org