omap2 clock: move 24xx-specific clock code from clock.c into clock24xx.c

In preparation for OMAP34xx clock code, move OMAP24xx-specific
clock code into a new file, 'clock24xx.c'. The remaining clock.c functions
are no longer static.  Also fix some style problems with the old code
identified by checkpatch.

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