dt-bindings: backlight: add TI LP8864/LP8866 LED-backlight drivers

dt-bindings: backlight: add TI LP8864/LP8866 LED-backlight drivers

Add bindings for Texas Instruments' LP8864/LP8866 LED-backlight drivers.
Note that multiple channels in these models are used for load-balancing and
brightness is controlled gobally, so from a user perspective it's only one
LED.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@siemens.com>
Link: https://lore.kernel.org/r/20241218210829.73191-2-alexander.sverdlin@siemens.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>