dts/bindings: Add gpio-nexus base binding

Add a base binding for GPIO nexus nodes.  This will get used by an GPIO
connectors.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

dts/bindings/gpio/arduino-header-r3.yaml

@@ -10,12 +10,8 @@ description: >
     This is a representation of GPIO pin nodes exposed as headers on Arduino R3
 
 inherits:
-    !include base.yaml
+    !include [gpio-nexus.yaml, base.yaml]
 
 properties:
     compatible:
       constraint: "arduino-header-r3"
-
-    gpio-map:
-      type: compound
-      category: required

dts/bindings/gpio/gpio-nexus.yaml

@@ -0,0 +1,28 @@
+#
+# Copyright (c) 2019, Linaro Limited
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+title: GPIO Nexus Base Structure
+
+description: >
+    This binding gives the base structure for all GPIO nexus nodes
+
+properties:
+    gpio-map:
+      type: compound
+      category: required
+
+    gpio-map-mask:
+      type: compound
+      category: optional
+
+    gpio-map-pass-thru:
+      type: compound
+      category: optional
+
+    "#gpio-cells":
+      type: int
+      category: required
+      description: Number of items to expect in a GPIO specifier