bindings: Add device-tree bindings for slate PIL driver

Add device-tree bindings documentation describing the properties
for the SLATE Peripheral Image Loader (PIL) driver.

Change-Id: I0739b3a73f71c145b705aabbc670c77bc6bd239e

bindings/pil/subsys-pil-slate.txt

@@ -0,0 +1,28 @@
+* Slate Subsystem Peripheral Image Loader
+
+subsys-pil-slate is a slate peripheral image loader (PIL) driver. It is
+used for loading the firmware images of the slate subsystems into memory
+and preparing the subsystem's processor to execute code. It's also
+responsible for shutting down the processor when it's not needed.
+
+Required properties:
+- compatible:	      Must be "qcom,pil-slate"
+- qcom,firmware-name: Base name of the firmware image.
+
+Optional properties:
+- qcom,slate2ap-status-gpio: GPIO used by the slate to indicate status to
+			     the apps.
+- qcom,ap2slate-status-gpio: GPIO used by the apps to indicate its status
+			     to slate.
+
+
+Example:
+	qcom,slate {
+		    compatible = "qcom,pil-slate";
+		    qcom,firmware-name = "slate-wear";
+		    /* GPIO inputs from blackghost */
+		    qcom,slate2ap-status-gpio = <&msm_gpio 97 0>;
+                    /* GPIO output to blackghost */
+                    qcom,ap2slate-status-gpio = <&msm_gpio 17 0>;
+	};
+