dt-bindings: backlight: Add MPS MP3309C

The Monolithic Power (MPS) MP3309C is a WLED step-up converter, featuring a
programmable switching frequency to optimize efficiency.
The brightness can be controlled either by I2C commands (called "analog"
mode) or by a PWM input signal (PWM mode).
This driver supports both modes.

For device driver details, please refer to:
- drivers/video/backlight/mp3309c_bl.c

The datasheet is available at:
- https://www.monolithicpower.com/en/mp3309c.html



Signed-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230925122609.78849-1-f.suligoi@asem.it


Signed-off-by: Lee Jones <lee@kernel.org>