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 <f.suligoi@asem.it>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20231116105319.957600-2-f.suligoi@asem.it
Signed-off-by: Lee Jones <lee@kernel.org>

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