[hrtimer] Change resolution storage to ktime_t format

Change the storage format of the per base resolution to ktime_t to
make it easier accessible in the hrtimers code.

Change the resolution from (NSEC_PER_SEC/HZ) to TICK_NSEC as Roman
pointed out. TICK_NSEC is closer to the real resolution.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>