ARM: davinci: convert to common sched_clock() implementation

Davinci has its own sched_clock() implementation, which gets in the
way of a single zImage. Moving to the common sched_clock framework
makes the code slightly cleaner.

Acked-by: Sekhar Nori <nsekhar@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>