clocksource: add API to force re-selection of the best clocksource

As the best clocksource is not selected till core boot completion,
only periodic tick timer works and it increases jiffies by one at
every tick updates. If interrupt is disabled more than one tick(10ms),
timer interrupts are missed and jiffies can't be updated at every
10ms and it can be behind the real time. So add API to force re-
selection of the best clocksource among registered clocksources so
that the best clocksource can be selected whenever it is available.

Change-Id: I481de3cdf1df8f0e35ed10aee7ab3882bf7a35b3
Signed-off-by: Se Wang (Patrick) Oh <sewango@codeaurora.org>
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>