dt-bindings: serial: add common properties schema for UART children

Properties should be defined in only one place, thus add new
serial-peripheral-props.yaml schema with definition of common properties
for UART-connected devices (children of UART controller): current-speed
and max-speed. The schema can be referenced by individual devices using
these properties.

Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Reviewed-by: Conor Dooley <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/serial/serial-peripheral-props.yaml

···
+# SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/serial/serial-peripheral-props.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Common Properties for Serial-attached Devices
+
+maintainers:
+  - Rob Herring <[e-mail]>
+  - Greg Kroah-Hartman <[e-mail]>
+
+description:
+  Devices connected over serial/UART, expressed as children of a serial
+  controller, might need similar properties, e.g. for configuring the baud
+  rate.
+
+properties:
+  max-speed:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description:
+      The maximum baud rate the device operates at.
+      This should only be present if the maximum is less than the slave
+      device can support.  For example, a particular board has some
+      signal quality issue or the host processor can't support higher
+      baud rates.
+
+  current-speed:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: |
+      The current baud rate the device operates at.
+      This should only be present in case a driver has no chance to know
+      the baud rate of the slave device.
+      Examples:
+        * device supports auto-baud
+        * the rate is setup by a bootloader and there is no way to reset
+          the device
+        * device baud rate is configured by its firmware but there is no
+          way to request the actual settings
+
+additionalProperties: true

Documentation/devicetree/bindings/serial/serial.yaml

···
       type: object
     then:
       additionalProperties: true
+      $ref: serial-peripheral-props.yaml#
       description:
         Serial attached devices shall be a child node of the host UART device
         the slave device is attached to. It is expected that the attached
···
         compatible:
           description:
             Compatible of the device connected to the serial port.
-
-        max-speed:
-          $ref: /schemas/types.yaml#/definitions/uint32
-          description:
-            The maximum baud rate the device operates at.
-            This should only be present if the maximum is less than the slave
-            device can support.  For example, a particular board has some
-            signal quality issue or the host processor can't support higher
-            baud rates.
-
-        current-speed:
-          $ref: /schemas/types.yaml#/definitions/uint32
-          description: |
-            The current baud rate the device operates at.
-            This should only be present in case a driver has no chance to know
-            the baud rate of the slave device.
-            Examples:
-              * device supports auto-baud
-              * the rate is setup by a bootloader and there is no way to reset
-                the device
-              * device baud rate is configured by its firmware but there is no
-                way to request the actual settings
 
       required:
         - compatible