max77819 backlight bindings

Valid node name : backlight

Optional properties :
 - current_map_id : currently, don't care
 - ramp_up_time_msec : ramp-up timer.
 - ramp_dn_time_msec : ramp-down timer.
 - pwm_input_use : It should be one of follow vaules
		0 - MAX77819_WLED_PWM_INPUT_DISABLE  ignoring WLEDPWM input
		1 - MAX77819_WLED_PWM_INPUT_CABC     CABC dimming signal
		2 - MAX77819_WLED_PWM_INPUT_EXTRA    extra control signal for higher resolution in brightness levels
 - pwm_id : PWM ID of AP.
		It will be used when "pwm_input_use == MAX77819_WLED_PWM_INPUT_EXTRA"
 - pwm_period_nsec : PWM period in nsec.
		It will be used when "pwm_input_use == MAX77819_WLED_PWM_INPUT_EXTRA"
Example:
	backlight {
		compatible = "maxim,max77819-wled";
		pwm_input_use = 2;
		pwm_id = 1;
		pwm_period_nsec = 1024*20;
	};
