Make mach/irqs.h optional for SPARSE_IRQ. With this change mach/irqs.h can
be removed by converting platforms over to sparse irq.

Platforms either need to set nr_irqs in their machine desc or all irqchips
used by a platform need to allocate their irq_descs. There cannot be a
mixture. Once this is done, the platforms can select SPARSE_IRQ. shmobile
does the latter, and mmp and pxa do the former.
Signed-off-by: Rob Herring <rob.herring@calxeda.com>