arm64: dts: imx8mm-beacon: Enable RTS-CTS on UART3

There is a header for a DB9 serial port, but any attempts to use
hardware handshaking fail.  Enable RTS and CTS pin muxing and enable
handshaking in the uart node.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>