For the AR933x UART, the serial clock is not the AHB clock, it's the

reference clock.  So use that instead.

sys/mips/atheros/uart_bus_ar933x.c

@@ -72,7 +72,7 @@ uart_ar933x_probe(device_t dev)
 	struct uart_softc *sc;
 	uint64_t freq;
 
-	freq = ar71xx_ahb_freq();
+	freq = ar71xx_refclk();
 
 	sc = device_get_softc(dev);
 	sc->sc_sysdev = SLIST_FIRST(&uart_sysdevs);

sys/mips/atheros/uart_cpu_ar933x.c

@@ -58,7 +58,7 @@ uart_cpu_getdev(int devtype, struct uart_devinfo *di)
 {
 	uint64_t freq;
 
-	freq = ar71xx_ahb_freq();
+	freq = ar71xx_refclk();
 
 	di->ops = uart_getops(&uart_ar933x_class);
 	di->bas.chan = 0;