OMAP2/3: clockdomain: remove unneeded .clkstctrl_reg, remove some direct CM register accesses

Reverse some of the effects of commit
84c0c39a ("ARM: OMAP4: PM: Make OMAP3
Clock-domain framework compatible for OMAP4").  On OMAP2/3, the
CM_CLKSTCTRL register is at a constant offset from the powerdomain's
CM instance.

Also, remove some of the direct CM register access from the
clockdomain code, moving it to the OMAP2/3 CM code instead.  The
intention here is to simplify the clockdomain code.  (The long-term
goal is to move all direct CM register access across the OMAP core
code to the appropriate cm*.c file.)

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Tested-by: Rajendra Nayak <rnayak@ti.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>