dt-bindings: fix warnings in xinpeng,xpp055c272.yaml

The reg property in the example caused following warnings:

xinpeng,xpp055c272.example.dts:20.17-27: Warning (reg_format): /example-0/dsi@ff450000/panel@0:reg: property has invalid length (4 bytes) (#address-cells == 2, #size-cells == 1)

xinpeng,xpp055c272.example.dt.yaml: Warning (pci_device_bus_num): Failed prerequisite 'reg_format'
xinpeng,xpp055c272.example.dt.yaml: Warning (i2c_bus_reg): Failed prerequisite 'reg_format'
xinpeng,xpp055c272.example.dt.yaml: Warning (spi_bus_reg): Failed prerequisite 'reg_format'
xinpeng,xpp055c272.example.dts:18.21-24.15: Warning (avoid_default_addr_size): /example-0/dsi@ff450000/panel@0: Relying on default #address-cells value
xinpeng,xpp055c272.example.dts:18.21-24.15: Warning (avoid_default_addr_size): /example-0/dsi@ff450000/panel@0: Relying on default #size-cells value

Added #address-cells and #size-cells to silence the warning.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200106181731.GA24294@ravnborg.org

Documentation/devicetree/bindings/display/panel/xinpeng,xpp055c272.yaml

@@ -35,6 +35,8 @@ additionalProperties: false
 examples:
   - |
     dsi@ff450000 {
+        #address-cells = <1>;
+        #size-cells = <0>;
         panel@0 {
             compatible = "xinpeng,xpp055c272";
             reg = <0>;