dt-bindings: clock: Update i.MX28 example

Update the i.MX28 clock example to align with MXS AUART binding doc to
avoid below build error:

Documentation/devicetree/bindings/clock/imx28-clock.example.dt.yaml:
  serial@8006a000: clocks: [[4294967295, 45]] is too short
Documentation/devicetree/bindings/clock/imx28-clock.example.dt.yaml:
  serial@8006a000: compatible: Additional items are not allowed
  ('fsl,imx23-auart' was unexpected)
Documentation/devicetree/bindings/clock/imx28-clock.example.dt.yaml:
  serial@8006a000: compatible: ['fsl,imx28-auart', 'fsl,imx23-auart']
  is too long
Documentation/devicetree/bindings/clock/imx28-clock.example.dt.yaml:
  serial@8006a000: 'dmas' is a required property
Documentation/devicetree/bindings/clock/imx28-clock.example.dt.yaml:
  serial@8006a000: 'dma-names' is a required property
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1597721685-9280-1-git-send-email-Anson.Huang@nxp.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/clock/imx28-clock.yaml

@@ -108,8 +108,10 @@ examples:
     };
 
     serial@8006a000 {
-        compatible = "fsl,imx28-auart", "fsl,imx23-auart";
+        compatible = "fsl,imx28-auart";
         reg = <0x8006a000 0x2000>;
-        interrupts = <112 70 71>;
+        interrupts = <112>;
+        dmas = <&dma_apbx 8>, <&dma_apbx 9>;
+        dma-names = "rx", "tx";
         clocks = <&clks 45>;
     };