sparc: Add sparc support for platform_get_irq()

This adds sparc support for platform_get_irq that in the normal case use
platform_get_resource() to get an irq. This standard approach fails for sparc as
there are no resources of type IORESOURCE_IRQ for irqs for sparc.

Cross platform drivers can then use this standard platform function and work on
sparc instead of having to have a special case for sparc.

Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>