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 <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
 			fifo-size = <32>;
 			reg-io-width = <4>;
 			reg-shift = <2>;
+			tx-threshold = <16>;
 		};
 
 		sysid: sysid@18001528 {