ath79: fix reference clock for RouterBoard 912UAG

This fixes reference clock frequency of RB912. 25 MHz frequency leads
to system clock running too fast, uptime incrementing too fast and
delays (like `sleep 10`) returning too early.

Board has quartz with NSK 3KHAA Z 40 000 marking.

Signed-off-by: Pavel Kamaev <pavel@kamaev.me>
(cherry picked from commit a716ac5564)

target/linux/ath79/dts/ar9342_mikrotik_routerboard-912uag-2hpnd.dts

@@ -147,7 +147,7 @@
 };
 
 &ref {
-	clock-frequency = <25000000>;
+	clock-frequency = <40000000>;
 };
 
 &spi {