dt-bindings: power: reset: Add qcom pon binding

The Power On device for Qcom PM 8xxx is a MFD supporting pwrkey and
resin along with the Android reboot-mode.

Add the binding describing this.
Suggested-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>

Documentation/devicetree/bindings/power/reset/qcom,pon.txt

+Qualcomm PON Device
+
+The Power On device for Qualcomm PM8xxx is MFD supporting pwrkey
+along with the Android reboot-mode.
+
+This DT node has pwrkey as sub node.
+
+Required Properties:
+-compatible: "qcom,pm8916-pon"
+-reg: Specifies the physical address of the pon register
+
+Optional subnode:
+-pwrkey: Specifies the subnode pwrkey and should follow the
+ qcom,pm8941-pwrkey.txt description.
+
+The rest of the properties should follow the generic reboot-mode description
+found in reboot-mode.txt
+
+Example:
+
+	pon@800 {
+		compatible = "qcom,pm8916-pon";
+
+		reg = <0x800>;
+		mode-bootloader = <0x2>;
+		mode-recovery = <0x1>;
+
+		pwrkey {
+			compatible = "qcom,pm8941-pwrkey";
+			interrupts = <0x0 0x8 0 IRQ_TYPE_EDGE_BOTH>;
+			debounce = <15625>;
+			bias-pull-up;
+			linux,code = <KEY_POWER>;
+		};
+	};