ARM: OMAP2+: PRM: move SoC specific init calls within a generic API

This gets rid of need for some exported driver APIs, and simplifies the
initialization of the PRM driver. Done in preparation to make PRM a
separate driver. The init data is now also passed to the SoC specific
implementations, allowing future expansion to add feature flags etc.

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