backlight: Add new lm3509 backlight driver

backlight: Add new lm3509 backlight driver

This is a general driver for LM3509 backlight chip of TI.
LM3509 is High Efficiency Boost for White LEDs and/or OLED Displays with
Dual Current Sinks. This driver supports OLED/White LED select, brightness
control and sub/main control.
The datasheet can be found at http://www.ti.com/product/lm3509.

Signed-off-by: Patrick Gansterer <paroga@paroga.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20240526105136.721529-3-paroga@paroga.com
Signed-off-by: Lee Jones <lee@kernel.org>