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

bindings/leds/backlight/qcom-spmi-wled.txt

@@ -10,9 +10,9 @@ platforms. The PMIC is connected to the host processor via SPMI bus.
 	Definition: should be one of the below.
 			"qcom,pmi8998-spmi-wled",
 			"qcom,pm8150l-spmi-wled",
-			"qcom,pm6150l-spmi-wled"
-			"qcom,pm660l-spmi-wled"
-			"qcom,pm7325b-spmi-wled"
+			"qcom,pm6150l-spmi-wled",
+			"qcom,pm660l-spmi-wled",
+			"qcom,pm7325b-spmi-wled",
 
 - reg
 	Usage:      required
@@ -158,6 +158,22 @@ platforms. The PMIC is connected to the host processor via SPMI bus.
 		        For details about IIO bindings see:
 			    Documentation/devicetree/bindings/iio/iio-bindings.txt
 
+- qcom,use-exp-dimming
+	Usage:      optional
+	Value type: <bool>
+	Definition: Specifies that exponential dimming lookup table values
+			    should be used.
+
+- qcom,exp-dimming-map
+	Usage:      optional
+	Value type: <u32 array>
+	Definition: It specifies a table of brightness values that can be programmed
+			    into WLED_SINK for exponential dimming which provides smooth brightness
+			    change. There should be exactly 256 values in the table and they should
+			    be at most 15 bits long. This table would be used only if the
+			    "qcom,use-exp-dimming" property is set. This feature is supported from
+			    PM7325B onwards.
+
 Following properties are for child subnodes that are needed for WLED preflash
 (or torch), flash and switch. These child subnodes can be specified only for
 PMICs that has WLED5 (e.g. PM8150L).