OMAP2/3 clock: add _omap2_clk_{read,write}_reg()

Create new static functions to read/write registers used in the struct
clk, _omap2_clk_{read,write}_reg().  A subsequent patch will use these
to ensure the correct register read/write instruction is used, which depends
on whether the clock registers are in the CM, PRM, or SCM.

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