omap2 clock: split out OMAP2/3 common defines, clksel rates

This patch is a collection of minor clock code changes in preparation for
the OMAP3 clock merge.  Specifically:

* Since general-purpose timer and GFX module L3 clksel rates are common to
  OMAP2/3, hoist those into asm/arch/clock.h.

* Since the CM_FCLKEN define is common to OMAP2/3, remove the OMAP24XX_
  prefix.

* Designate the clock transaction code in clock.c as being OMAP2-specific.

* Fix a potential null pointer dereference that could occur when debugging
  is enabled in omap2_init_clksel_parent().  Clocks with no initially-defined
  parent (as is possible for clksel clocks) will trigger an oops due to a
  buggy conditional.

* Clean up omap2_wait_clock_ready().

* Remove unused struct dpll_params auto_idle* fields.

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