gpio: Introduce mcux igpio shim driver

Adds a new gpio driver for the NXP i.MX family of SoCs. Read, write,
configure, and callback API functions are all implemented.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>

dts/bindings/gpio/nxp,imx-gpio.yaml

@@ -0,0 +1,43 @@
+#
+# Copyright (c) 2017, NXP
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: i.MX GPIO
+version: 0.1
+
+description: >
+    This is a representation of the i.MX GPIO nodes
+
+properties:
+  - compatible:
+      type: string
+      category: required
+      description: compatible strings
+      constraint: "nxp,imx-gpio"
+
+  - 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
+...