dt-bindings: backlight: mp3309c: Remove two required properties

The two properties:

- max-brightness
- default brightness

are not really required, so they can be removed from the "required"
section.
The "max-brightness" is no longer used in the current version
of the driver (it was used only in the first version).
The "default-brightness", if omitted in the DT, is managed by the
device driver, using a default value. This value depends on the dimming
mode used:

- for the "analog mode", via I2C commands, this value is fixed by
hardware (=31)
- while in case of pwm mode the default used is the last value of the
brightness-levels array.

Also the brightness-levels array is not required:

- in "analog mode", via I2C commands, the brightness-level array is
fixed by hardware (0..31).;
- in pwm dimming mode, the driver uses a default array of 0..255 and
the "default-brightness" is the last one, which is "255"

NOTE: there are no compatibility problems with the previous version,
since the device driver has not yet been included in any kernel.
Only this dt-binding yaml file is already included in the current
v6.7.0-rc1 kernel version.
No developer may have used it.

Other changes:

- improve the backlight working mode description, in the "description"
section
- update the example, removing the "max-brightness" and introducing the
"brightess-levels" property

Signed-off-by: Flavio Suligoi <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Reviewed-by: Daniel Thompson <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/leds/backlight/mps,mp3309c.yaml

···
   programmable switching frequency to optimize efficiency.
   It supports two different dimming modes:
 
-  - analog mode, via I2C commands (default)
-  - PWM controlled mode.
+  - analog mode, via I2C commands, as default mode (32 dimming levels)
+  - PWM controlled mode (optional)
 
   The datasheet is available at:
   https://www.monolithicpower.com/en/mp3309c.html
···
 required:
   - compatible
   - reg
-  - max-brightness
-  - default-brightness
 
 unevaluatedProperties: false
 
···
             compatible = "mps,mp3309c";
             reg = <0x17>;
             pwms = <&pwm1 0 3333333 0>; /* 300 Hz --> (1/f) * 1*10^9 */
-            max-brightness = <100>;
-            default-brightness = <80>;
+            brightness-levels = <0 4 8 16 32 64 128 255>;
+            default-brightness = <6>;
             mps,overvoltage-protection-microvolt = <24000000>;
         };
     };