ARM: OMAP2+: PRM: determine PRM base address from device tree

There is no need to provide the PRM base address through a low-level API
from the low-level IO init, as this information is available through DT.
Re-routed the parsing function to be called from the PRM drivers also to
simplify the implementation under io.c.

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