ARM: OMAP: hwmod: add support for selecting mpu_irq for each wakeup pad

By default all registered pads will trigger mpu_irqs[0]. Now there is
an API for selecting used mpu_irq on pad basis, which can be used to
trigger different irq handlers for different pads in the same hwmod.
Each pad that requires its interrupt to be re-routed this way must
have a separate call to omap_hwmod_pad_route_irq(hwmod, pad, irq).

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: moved fn to omap_hwmod.c; separated fn from mux scan_wakeups
 changes; added kerneldoc]
Signed-off-by: Paul Walmsley <paul@pwsan.com>