omap2: convert clock.h to use symbolic register bit defines

Convert magic numbers in the enable_bit member of the struct clk
definitions in clock.h to use the new-style register bit defines from
{prm,cm}_regbits_24xx.h.

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