dts: pulpino: Add device tree support for GPIO controller

Add the needed bits to get device tree support for the GPIO controller
on the Zedboard-Pulpino.  This will allow us to move LED & button info
into the board.dts.

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

dts/bindings/gpio/pulp,pulpino-gpio.yaml

@@ -0,0 +1,45 @@
+#
+# Copyright (c) 2018, Linaro Inc.
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: PULPINO GPIO Controller
+id: pulp,pulpino-gpio
+version: 0.1
+
+description: >
+    This is a representation of the Pulpino GPIO node
+
+properties:
+    compatible:
+      type: string
+      category: required
+      description: compatible strings
+      constraint: "pulp,pulpino-gpio"
+      generation: define
+
+    reg:
+      type: int
+      description: mmio register space
+      generation: define
+      category: required
+
+    interrupts:
+      type: compound
+      category: required
+      description: required interrupts
+      generation: define
+
+    label:
+      type: string
+      category: required
+      description: Human readable string describing the device (used by Zephyr for API name)
+      generation: define
+
+cell_string: GPIO
+
+"#cells":
+  - pin
+  - flags
+...