backlight: qcom-spmi-wled: 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: Ifd2dd382bf3e1274a5b994e8d83b87833c975970
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>