drivers: timer: Improve the accuracy of the MCUX OS Timer

1. The sys_clock_idle_exit function could be invoked multiple
   times. Hence add code so that is counter is stopped and the
   OS Timer is initialized once.
2. Reset the OS Timer when exiting low power modes where
   the OS Timer loses its state
3. Improve the cycles conversion algorithm. Round to the nearest
   microsecond when converting from ticks to microsecond rather
   than always rounding up to the next highest value.

Signed-off-by: Mahesh Mahadevan <mahesh.mahadevan@nxp.com>