omap2 clock: generalize clock enable upon framework initialization

The OMAP2 clock framework enables a few clocks by hand upon initialization.
OMAP3430 will have a different set of clocks to enable upon init.  So,
generalize this code by adding a new clock flag, ENABLE_ON_INIT, to mark
clocks that should be enabled on init; and add a new framework-wide function,
clk_enable_init_clocks(), that can be called from generalized init code.

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