use trim values to set mixer:scaler.offset

clamp mixer output offset to [-.2,.2] ([-2000, 2000] in mixer file)

add 8 main PWM trim parameters

add long desc to parameters and bump minor parameter version

src/modules/px4iofirmware/mixer.cpp

@@ -212,6 +212,13 @@ mixer_tick(void)
 		r_status_flags &= ~(PX4IO_P_STATUS_FLAGS_FAILSAFE);
 	}
 
+	/*
+	 * Set simple mixer trim values
+	 * (there should be a "dirty" flag to indicate that r_page_servo_control_trim has changed)
+	 */
+	mixer_group.set_trims(r_page_servo_control_trim, PX4IO_SERVO_COUNT);
+
+
 	/*
 	 * Run the mixers.
 	 */