arm64: dts: imx8mq: Use 'dsi' as node name

Use 'dsi' as node name to avoid the following schema warning:

imx8mq-evk.dtb: mipi-dsi@30a00000: $nodename:0: 'mipi-dsi@30a00000' does not match '^dsi(@.*)?$'
From schema: Documentation/devicetree/bindings/display/bridge/nwl-dsi.yaml

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>