Make clock optional on uart nodes. It is a FreeBSD-specific extention,

and won't be present in vendor supplied dts files.

sys/dev/uart/uart_bus_fdt.c

@@ -97,9 +97,12 @@ uart_fdt_get_clock(phandle_t node, pcell_t *cell)
 {
 	pcell_t clock;
 
+	/*
+	 * clock-frequency is a FreeBSD-specific hack. Make its presence optional.
+	 */
 	if ((OF_getprop(node, "clock-frequency", &clock,
 	    sizeof(clock))) <= 0)
-		return (ENXIO);
+		clock = 0;
 
 	if (clock == 0)
 		/* Try to retrieve parent 'bus-frequency' */