arm64: mvebu: armada-ap806.dtsi: Add clock-frequency to UART DT node

The clock frequency needs to be provided in the DT. Otherwise the driver
won't start in U-Boot.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Nadav Haklai <nadavh@marvell.com>
Cc: Neta Zur Hershkovits <neta@marvell.com>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Omri Itach <omrii@marvell.com>
Cc: Igal Liberman <igall@marvell.com>
Cc: Haim Boot <hayim@marvell.com>
Cc: Hanna Hawa <hannah@marvell.com>

arch/arm/dts/armada-ap806.dtsi

@@ -202,6 +202,7 @@
 				reg-io-width = <1>;
 				clocks = <&ap_syscon 3>;
 				status = "disabled";
+				clock-frequency = <200000000>;
 			};
 
 			uart1: serial@512100 {