ARM: OMAP: Timer32K: Move 32k-based sched_clock() to common code

Since 32k timer code is moving to OMAP1 specific dir, move the
32k-based sched_clock() into common code where it is based on the 32k
sync counter and can be used even when using MPU timer.

While moving, change the ticks-to-nsecs conversion to use the helper
functions provided by clocksource.h.

Also removed the unused ticks_to_usec, leaving only ticks_to_nsec.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>