dts: bindings: Add yaml files for PHY subsystem

Add yaml file for initial support of USB PHYs.
It adds also yaml files for STM32 USBPHYC and
USB controllers with embedded PHY.

PHY refers to the physical layer which is used
to connect a device to the physical medium e.g.
USB controller.

Origin: original

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

dts/bindings/phy/phy.yaml

@@ -0,0 +1,29 @@
+#
+# Copyright (c) 2018, Yannis Damigos
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: PHY Base Structure
+version: 0.1
+
+description: >
+    This binding gives the base structures for all PHY providers
+
+properties:
+    compatible:
+      type: string
+      category: required
+      description: compatible strings
+      generation: define
+    "#phy-cells":
+      type: int
+      category: required
+      description: Number of cells in a PHY provider. The meaning those
+                   cells is defined by the binding for the phy node.
+    label:
+      type: string
+      category: required
+      description: Human readable string describing the device (used by Zephyr for API name)
+      generation: define
+...

dts/bindings/phy/st,stm32-usbphyc.yaml

@@ -0,0 +1,28 @@
+#
+# Copyright (c) 2018, Yannis Damigos
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: STM32 USB HS PHY
+version: 0.1
+
+description: >
+    This binding gives a base representation of the STM32 USB HS PHY controller
+
+inherits:
+    !include phy.yaml
+
+properties:
+    compatible:
+      constraint: "st,stm32-usbphyc"
+
+    reg:
+      type: array
+      description: address and length of the usb phy control register set
+      generation: define
+      category: required
+
+    "phy-cells":
+      description: should be 0
+...

dts/bindings/phy/usb-nop-xceiv.yaml

@@ -0,0 +1,23 @@
+#
+# Copyright (c) 2018, Yannis Damigos
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+---
+title: NOP USB Transceiver
+version: 0.1
+
+description: >
+    This binding is to be used by all the usb transceivers which are built-in
+    with USB IP
+
+inherits:
+    !include phy.yaml
+
+properties:
+    compatible:
+      constraint: "usb-nop-xceiv"
+
+    "phy-cells":
+      description: should be 0
+...