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>

arch/arm64/boot/dts/freescale/imx8mm-beacon-baseboard.dtsi

@@ -278,6 +278,7 @@ &uart3 {
 	pinctrl-0 = <&pinctrl_uart3>;
 	assigned-clocks = <&clk IMX8MM_CLK_UART3>;
 	assigned-clock-parents = <&clk IMX8MM_SYS_PLL1_80M>;
+	uart-has-rtscts;
 	status = "okay";
 };
 
@@ -386,6 +387,8 @@ pinctrl_uart3: uart3grp {
 		fsl,pins = <
 			MX8MM_IOMUXC_ECSPI1_SCLK_UART3_DCE_RX	0x40
 			MX8MM_IOMUXC_ECSPI1_MOSI_UART3_DCE_TX	0x40
+			MX8MM_IOMUXC_ECSPI1_MISO_UART3_DCE_CTS_B	0x40
+			MX8MM_IOMUXC_ECSPI1_SS0_UART3_DCE_RTS_B	0x40
 		>;
 	};