omap2 clock: use the struct clk round_rate field for clksel rate rounding code

The OMAP clock framework struct clk contains a field, .round_rate,
that is intended to contain a function pointer to rate rounding code.
Take advantage of it by converting all clksel clocks' round_rate field
to point to omap2_clksel_round_rate().  Rename the existing
omap2_clksel_round_rate() to omap2_clksel_round_rate_div(), to signify
that it returns divisor information as well, and set up
omap2_clksel_round_rate() as a wrapper for the clock framework to use.
Simplify omap2_clk_round_rate() accordingly.

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