timekeeping: Use inject_offset in warp_clock

timekeeping: Use inject_offset in warp_clock

When warping the clock (from a local time RTC), use
timekeeping_inject_offset() to atomically add the offset.

This avoids any minor time error caused by the delay between
reading the time, and then setting the adjusted time.

Signed-off-by: John Stultz <john.stultz@linaro.org>