arm64: dts: imx8mm: Configure DMA on UART2

arm64: dts: imx8mm: Configure DMA on UART2

UART2 is often used as the console, so the DMA was likely left
off on purpose, since it's recommended to not use the DMA on the
console. Because, the driver checks to see if the UART is used for
the console when determining if it should initialize DMA, it
should be safe to enable DMA on UART2 for all users.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>