dts: bindings: add modem binding for u-blox SARA-R4

The SARA-R4 series modules from U-Blox are size-optimized LTE-M /
NB-IoT and EGPRS modules designed for low power consumption and
longer battery life.

The binding identifies the UART device, power GPIO and reset GPIO
lines.

Signed-off-by: Michael Scott <[email protected]>

Author: mike-scott

dts/bindings/modem/ublox,sara-r4.yaml

@@ -0,0 +1,37 @@
+#
+# Copyright (c) 2019, Foundries.io
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: u-blox SARA-R4 modem
+version: 0.1
+
+description: >
+    This is a representation of the u-blox SARA-R4 modem.
+
+inherits:
+    !include uart-device.yaml
+
+properties:
+    compatible:
+      type: string
+      category: required
+      constraint: "ublox,sara-r4"
+      generation: define
+
+    label:
+      type: string
+      category: required
+      generation: define
+
+    mdm-power-gpios:
+      type: compound
+      category: required
+      generation: define, use-prop-name
+
+    mdm-reset-gpios:
+      type: compound
+      category: required
+      generation: define, use-prop-name
+...