msm: timer: Support sched_clock()

Now that sched_clock is mandatory on ARM it's simple to add
sched_clock support to the MSM timer code. Add it so that we get
more accurate sched_clock output than the jiffies based version
that's provided by default.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Acked-by: David Brown <davidb@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>