PRM/CM: Convert existing code to use PRM/CM RMW functions

Convert existing code that reads, modifies, and writes back CM/PRM
register values to use the rmw functions introduced in the previous
patch.  This code should eventually disappear once clockdomain handling
is integrated into the 24xx clock framework.

Also restructure arch/arm/mach-omap2/prcm.c slightly while we
are here.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>