arm64: dts: imx8qm-ss-dma: Pass lpuart dma-names

Per fsl-lpuart.yaml, when the 'dmas' property is used 'dma-names' should
also be present.

Pass the lpuart 'dma-names' property to fix the following schema
warnings:

imx8dxl-evk.dtb: serial@5a060000: dma-names:0: 'rx' was expected
	from schema $id: http://devicetree.org/schemas/serial/fsl-lpuart.yaml#
imx8dxl-evk.dtb: serial@5a060000: dma-names:1: 'tx' was expected
	from schema $id: http://devicetree.org/schemas/serial/fsl-lpuart.yaml#
imx8dxl-evk.dtb: serial@5a060000: Unevaluated properties are not allowed ('dma-names' was unexpected)
	from schema $id: http://devicetree.org/schemas/serial/fsl-lpuart.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/imx8qm-ss-dma.dtsi

@@ -117,21 +117,25 @@ &flexcan3 {
 &lpuart0 {
 	compatible = "fsl,imx8qm-lpuart", "fsl,imx8qxp-lpuart";
 	dmas = <&edma2 13 0 0>, <&edma2 12 0 1>;
+	dma-names = "rx","tx";
 };
 
 &lpuart1 {
 	compatible = "fsl,imx8qm-lpuart", "fsl,imx8qxp-lpuart";
 	dmas = <&edma2 15 0 0>, <&edma2 14 0 1>;
+	dma-names = "rx","tx";
 };
 
 &lpuart2 {
 	compatible = "fsl,imx8qm-lpuart", "fsl,imx8qxp-lpuart";
 	dmas = <&edma2 17 0 0>, <&edma2 16 0 1>;
+	dma-names = "rx","tx";
 };
 
 &lpuart3 {
 	compatible = "fsl,imx8qm-lpuart", "fsl,imx8qxp-lpuart";
 	dmas = <&edma2 19 0 0>, <&edma2 18 0 1>;
+	dma-names = "rx","tx";
 };
 
 &i2c0 {