ARM: OMAP: TimerMPU: Remove MPU-timer based sched_clock()

Remove MPU-timer based sched_clock() in favor of the common one based
on 32k sync timer which works across all OMAP1/2/3 platforms.

Using 32k based one also gives a valid sched_clock() very early in the
boot process.

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