dt-bindings: serial: mxs: Fix compatible list

dt-bindings: serial: mxs: Fix compatible list

imx23 and imx28 are SoCs from the same family (mxs) and they share the
same AUART block, so the same programming model.

imx23 is the first member of this family. It had an AUART DMA erratum.
imx28 is the second member of this family and has this erratum fixed.

imx28.dtsi uses the following description:

compatible = "fsl,imx28-auart", "fsl,imx23-auart";

Make it valid so that the following schema warning could be avoided:

imx28-apx4devkit.dtb: serial@8006a000: compatible: ['fsl,imx28-auart', 'fsl,imx23-auart'] is too long
	from schema $id: http://devicetree.org/schemas/serial/fsl-mxs-auart.yaml#

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230921133733.224602-1-festevam@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>