ARM: OMAP2+: PRM: add support for initializing PRCM clock modules from DT

This patch provides top level functionality for the DT clock initialization.
Clock tree is initialized hierarchically starting from IP modules (CM/PRM/PRCM)
going down towards individual clock nodes, and finally initializing
clockdomains once all the clocks are ready.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>