ARM: OMAP4: PRM: use PRCM interrupt handler

Use the new PRCM interrupt handler code on OMAP4 systems.

The OMAP code will need to be converted to use sparse IRQs for this
to work.  Until that time, the following message will appear on boot:

PRCM: failed to allocate irq descs: -12

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
[paul@pwsan.com: split this from a previous patch to this patch; call
 omap4xxx_prcm_init() during init; write trivial commit log]
Signed-off-by: Paul Walmsley <paul@pwsan.com>