net: wifi_mgmt: Configure ps wakeup mode

'ps_wakeup_mode' is used to set the wake up interval
to either 'dtim or 'listen_interval'. By default the
sta wakes up every dtim interval.
With 'ps_wakeup_mode' set to 'listen_interval' sta
will wake up for every listen interval period configured.

Signed-off-by: Ajay Parida <ajay.parida@nordicsemi.no>