OMAP2+ clock: clean up clkt_clksel.c

This patch cleans up arch/arm/mach-omap2/clkt_clksel.c.  It:

- makes several functions static that are not called outside the file;

- adds documentation;

- makes some code paths easier to read (hopefully), by breaking up
  compound statements and removing redundant checks;

- converts some pr_err()s that indicate clock tree data problems into WARN()s,
  so they are more likely to be noticed;

- and moves omap2_clk_round_rate() back into mach-omap2/clock.c, its proper
  home, since it is not clksel-specific.

Signed-off-by: Paul Walmsley <paul@pwsan.com>