dt-bindings: backlight: Add support for exponential dimming

Add support to enable writing a lookup table of brightness values
into WLED_SINK peripheral to enable more granular control over
brightness, to help end user see brightness change smoothly
instead of in visible steps.

Change-Id: Ieecd52f2af5f8b3d2daea8ff1cd5e8b51e2ad5c6