modem: modem_cellular: streamline instance configuration with new macro

Introduced `MODEM_CELLULAR_DEFINE_INSTANCE` macro to streamline
the definition and initialization of modem cellular instances.

This macro enables passing parameters that vary among modems, including:
`power_pulse_duration_ms`, `reset_pulse_duration_ms`, `startup_time_ms`,
`shutdown_time_ms`, `autostarts`, `init_chat_script`, `dial_chat_script`
and `periodic_chat_script`.

This will reduce the number of redundant lines in the code, aiming for
improved maintainability.

Signed-off-by: Pisit Sawangvonganan <[email protected]>

Author: ndrs-pst