time: source: updated the clk source to be atomic

The clocksource::tc field is from now on an atomic type (atomic64_t).
This to ensure that none can ever read a mashed up value when trying to
get the tick counter of a timer (which is usually updated by some IRQ).

Signed-off-by: Michel Megens <dev@michelmegens.net>
Merge branch 'time'

* Atomic type for clocksource::tc