drivers: counter: adopt SHELL_HELP

[kartben]

Adopt SHELL_HELP macro for counter_timer_shell

uart:~$ timer --help
timer - Timer commands
Subcommands:
  periodic  : Start a periodic timer with specified interval
              Usage: periodic <timer_instance_node_id> <time_in_us>
  oneshot   : Set a one-shot alarm on specified channel
              Usage: oneshot <timer_instance_node_id> <channel_id> <time_in_us>
  freerun   : Start a timer in free-running mode
              Usage: freerun <timer_instance_node_id>
  stop      : Stop a timer
              Usage: stop <timer_instance_node_id>

before was looking like this:

uart:~$ timer --help
timer - Timer commands
Subcommands:
  periodic  : timer periodic <timer_instance_node_id> <time_in_us>
  oneshot   : timer oneshot <timer_instance_node_id> <channel_id> <time_in_us>
  freerun   : timer freerun <timer_instance_node_id>
  stop      : timer stop <timer_instance_node_id>

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[kartben]

@nordic-krch looking good to you?