dt-bindings: pwm-backlight: Move brightness-levels to optional

The patch 'backlight: pwm_bl: compute brightness of LED linearly to
human eye' introduced a default brightness-levels table that is used
when brightness-levels is not available in the dts. So move
brightness-levels and default-brightness-level to be optional.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt

@@ -3,13 +3,6 @@ pwm-backlight bindings
 Required properties:
   - compatible: "pwm-backlight"
   - pwms: OF device-tree PWM specification (see PWM binding[0])
-  - brightness-levels: Array of distinct brightness levels. Typically these
-      are in the range from 0 to 255, but any range starting at 0 will do.
-      The actual brightness level (PWM duty cycle) will be interpolated
-      from these values. 0 means a 0% duty cycle (darkest/off), while the
-      last value in the array represents a 100% duty cycle (brightest).
-  - default-brightness-level: the default brightness level (index into the
-      array defined by the "brightness-levels" property)
   - power-supply: regulator for supply voltage
 
 Optional properties:
@@ -21,6 +14,14 @@ Optional properties:
                           and enabling the backlight using GPIO.
   - pwm-off-delay-ms: Delay in ms between disabling the backlight using GPIO
                       and setting PWM value to 0.
+  - brightness-levels: Array of distinct brightness levels. Typically these
+                       are in the range from 0 to 255, but any range starting at
+                       0 will do. The actual brightness level (PWM duty cycle)
+                       will be interpolated from these values. 0 means a 0% duty
+                       cycle (darkest/off), while the last value in the array
+                       represents a 100% duty cycle (brightest).
+  - default-brightness-level: The default brightness level (index into the
+                              array defined by the "brightness-levels" property).
   - num-interpolated-steps: Number of interpolated steps between each value
                             of brightness-levels table. This way a high
                             resolution pwm duty cycle can be used without