nios2: dts: 10m50: Add tx-threshold parameter

The tx-threshold parameter sets the TX FIFO low water threshold
trigger for the Altera 16550-FIFO32 soft IP.
Signed-off-by: Thor Thayer <tthayer@opensource.altera.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/nios2/boot/dts/10m50_devboard.dts

@@ -83,6 +83,7 @@ a_16550_uart_0: serial@18001600 {
 			fifo-size = <32>;
 			reg-io-width = <4>;
 			reg-shift = <2>;
+			tx-threshold = <16>;
 		};
 
 		sysid: sysid@18001528 {