omap2 clock: convert omap2_clksel_to_divisor and omap2_divisor_to_clksel to use new...

Convert omap2_clksel_to_divisor() and omap2_divisor_to_clksel() to use
the new struct clksel and struct clksel_rate data.  Add
omap2_get_clksel_by_parent() utility code, called by both functions,
to return the appropriate struct clksel for a given (struct clk, parent
struct clk) combination.

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