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>