ARM: OMAP2+: clock: move clock provider infrastructure to clock driver

Splits the clock provider init out of the PRM driver and moves it to
clock driver. This is needed so that once the PRCM drivers are separated,
they can logically just access the clock driver not needing to go through
common PRM code. This would be wrong in the case of control module for
example.

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