ARM: OMAP3+: PRM: register interrupt information from DT

Allow the PRM interrupt information to be picked up from device tree.
OMAP3 may use legacy boot and needs to be compatible with old dtbs
(without interrupt populated), for these, we use the value which is
pre-populated.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>