dt-bindings: clock: Update i.MX23 example

Update the i.MX23 clock example to align with MXS AUART binding doc to
avoid below build error:

Documentation/devicetree/bindings/clock/imx23-clock.example.dt.yaml:
   serial@8006c000: clocks: [[4294967295, 32]] is too short
Documentation/devicetree/bindings/clock/imx23-clock.example.dt.yaml:
   serial@8006c000: 'dmas' is a required property
Documentation/devicetree/bindings/clock/imx23-clock.example.dt.yaml:
    serial@8006c000: 'dma-names' is a required property

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1597721685-9280-2-git-send-email-Anson.Huang@nxp.com


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