ARC: [plat-arcfpga] DT arc-uart bindings change: "baud" => "current-speed"

Per Grant's review comment - driver changes via tty tree
Signed-off-by: NVineet Gupta <vgupta@synopsys.com>

arch/arc/boot/dts/angel4.dts

@@ -48,7 +48,7 @@ arcuart0: serial@c0fc1000 {
 			reg = <0xc0fc1000 0x100>;
 			interrupts = <5>;
 			clock-frequency = <80000000>;
-			baud = <115200>;
+			current-speed = <115200>;
 			status = "okay";
 		};
 	};