arm64: dts: mba8mx: Remove unneeded comments

The bootargs line is already commented out and the comment of UART4
does not apply, as uart4 node is not described.

Remove both comments to make the dtsi cleaner.

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