timekeeping: Store cycle_last value in timekeeper struct as well
For implementing a shadow timekeeper and a split calculation/update
region we need to store the cycle_last value in the timekeeper and
update the value in the clocksource struct only in the update region.
Add the extra storage to the timekeeper.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: John Stultz <john.stultz@linaro.org>