omap2 clock: separate clksel set_rate code into its own function

Simplify omap2_clk_set_rate() by separating out the clksel set_rate code into
its own function, omap2_clksel_set_rate().  Clksel clocks use this function
by placing its function pointer in their .set_rate fields.  Convert
existing clksel clocks accordingly.

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