driver: arcv2_timer0: fixes and optimize the timer driver

* MIN_DELAY: 1024 -> 512
* optimzie some code sequence
* fix a bug in setting the new timer limit value
   * case: before set limit register with new value,
     if counter rolls back to 0, the limit value should be
     adjusted.

Signed-off-by: Wayne Ren <wei.ren@synopsys.com>