db820c: add dummy clock for uart node

The missing clock causes serial_msm driver probe to fail.
Added a dummy node so the probe succeeds, as the clock init
currently in db820c is empty.

Fixes: 11d59fe5374a ("serial: serial_msm: fail probe if settings clocks fails")

Signed-off-by: Ramon Fried <ramon.fried@gmail.com>

arch/arm/dts/dragonboard820c.dts

@@ -50,6 +50,7 @@
 		blsp2_uart1: serial@75b0000 {
 			compatible = "qcom,msm-uartdm-v1.4", "qcom,msm-uartdm";
 			reg = <0x75b0000 0x1000>;
+			clock = <&gcc 4>;
 		};
 
 		sdhc2: sdhci@74a4900 {