leds: add DT binding for Qualcomm PM8941 WLED block

This adds device tree binding documentation for the WLED ('White' LED)
block on Qualcomm's PM8941 PMICs.
Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

Documentation/devicetree/bindings/leds/leds-pm8941-wled.txt

+Binding for Qualcomm PM8941 WLED driver
+
+Required properties:
+- compatible: should be "qcom,pm8941-wled"
+- reg: slave address
+
+Optional properties:
+- label: The label for this led
+  See Documentation/devicetree/bindings/leds/common.txt
+- linux,default-trigger: Default trigger assigned to the LED
+  See Documentation/devicetree/bindings/leds/common.txt
+- qcom,cs-out: bool; enable current sink output
+- qcom,cabc: bool; enable content adaptive backlight control
+- qcom,ext-gen: bool; use externally generated modulator signal to dim
+- qcom,current-limit: mA; per-string current limit; value from 0 to 25
+	default: 20mA
+- qcom,current-boost-limit: mA; boost current limit; one of:
+	105, 385, 525, 805, 980, 1260, 1400, 1680
+	default: 805mA
+- qcom,switching-freq: kHz; switching frequency; one of:
+	600, 640, 685, 738, 800, 872, 960, 1066, 1200, 1371,
+	1600, 1920, 2400, 3200, 4800, 9600,
+	default: 1600kHz
+- qcom,ovp: V; Over-voltage protection limit; one of:
+	27, 29, 32, 35
+	default: 29V
+- qcom,num-strings: #; number of led strings attached; value from 1 to 3
+	default: 2
+
+Example:
+
+pm8941-wled@d800 {
+	compatible = "qcom,pm8941-wled";
+	reg = <0xd800>;
+	label = "backlight";
+
+	qcom,cs-out;
+	qcom,current-limit = <20>;
+	qcom,current-boost-limit = <805>;
+	qcom,switching-freq = <1600>;
+	qcom,ovp = <29>;
+	qcom,num-strings = <2>;
+};