dts: bindings: provide generic yaml description for spi-devices

Provide generic yaml description for spi-devices such as sensors.

Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>

dts/bindings/spi/spi-device.yaml

@@ -0,0 +1,37 @@
+#
+# Copyright (c) 2018, I-SENSE group of ICCS
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: SPI Device Base Structure
+id: spi-device
+version: 0.1
+
+description: >
+    This binding gives the base structures for all spi devices
+
+parent:
+    bus: spi
+
+properties:
+    compatible:
+      type: string
+      category: required
+      description: compatible strings
+    reg:
+      type: array
+      description: Chip select address of device
+      generation: define
+      category: required
+    spi-max-frequency:
+      type: u32
+      category: required
+      description: Maximum clock frequency of device's SPI interface in Hz
+      generation: define
+    label:
+      type: string
+      category: required
+      description: Human readable string describing the device (used by Zephyr for API name)
+      generation: define
+...

dts/bindings/spi/spi.yaml

@@ -11,6 +11,9 @@ version: 0.1
 description: >
     This binding gives the base structures for all SPI devices
 
+child:
+    bus: spi
+
 properties:
     "#address-cells":
       type: int