px4fmu: support all actuator control groups

This update of px4fmu PWM driver allows to use all 4 actuator control groups and do mixing properly. It's useful e.g. for controlling camera gimbal or manual control passthrough (groups 2 and 3). Required uORB topics subscribed only if needed depending on mixer (using mixer->groups_required() method).

Still no idea how to select `actuator_outputs` topic. By now, using old approach based on primary PWM device (first driver that captured `/dev/pwm_output` is primary), but it's not correct. Primary output device can be not PWM device (e.g. mkblctrl). Need to find a way how to define actuator outputs group for each driver. Any ideas?