counter: mchp_xec: update driver for counter tests

The counter driver tests have been updated so the driver needs
to be updated too.

() Test expects a free running counter.
() Test expects any alarms cannot be set beyond the top value.
() Also, the counter only triggers interrupts when counter reaches
   zero (as configured as counting down), it can be do relative
   alarms. So return -ENOTSUP when absolute alarms are requested.
() The test expects the callback to be removed once alarm is
   triggered. Implement this too.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>