dt-bindings: serial: Describe ARM dcc interface

Debug Communication Channel (DCC) provides the way how to pass data between
target CPU and host via JTAG interface. Every CPU has own interface for
communication via dbgdtrtx_el0 and dbgdtrrx_el0 registers.
Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/9d7e85914eb1cdb313b28cb019093a84dd9b4773.1703167505.git.michal.simek@amd.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/arm,dcc.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serial/arm,dcc.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: ARM DCC (Data communication channel) serial emulation
+
+maintainers:
+  - Michal Simek <michal.simek@amd.com>
+
+description: |
+  ARM DCC (Data communication channel) serial emulation interface available
+  via JTAG can be also used as one of serial line tightly coupled with every
+  ARM CPU available in the system.
+
+properties:
+  compatible:
+    const: arm,dcc
+
+required:
+  - compatible
+
+additionalProperties: false
+
+examples:
+  - |
+    serial {
+      compatible = "arm,dcc";
+    };