dt-bindings: clock: Update i.MX28 example

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.com
Signed-off-by: Rob Herring <robh@kernel.org>