irq_domain/microblaze: Convert microblaze to use irq_domains

This patch converts Microblaze to use the irq_domain remapper and get
away from hard coding the offset between hwirq number and the linux irq
number space.  This also paves the way for multiple interrupt controllers.

v2: Don't enable SPARSE_IRQ and keep NR_IRQS set to 33

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Michal Simek <monstr@monstr.eu>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: John Williams <john.williams@petalogix.com>
Cc: John Linn <john.linn@xilinx.com>