ath79: rb912: fix pll init issues

It was reported that some rb912 boards (ar934x) have issues with some ethernet speeds.
Investigation shows that the board failed to adapt the ethernet pll values as shown here:

[    5.284359] ag71xx 19000000.eth: failed to read pll-handle property

added custom prints in code and triggering a link switch:

[   62.821446] Atheros AG71xx: fast reset
[   62.826442] Atheros AG71xx: update pll 2
[   62.830494] Atheros AG71xx: no pll regmap!

Comparison with another very similar board (rb922 - QCA955x) showed a missing
reference clock frequency in dts, which seems to cause a pll init issue.
Unfortunately, no errors are printed when this occurs.

Adding the frequency property fixes the pll init as it can be parsed now
by the ethernet driver.

[   55.861407] Atheros AG71xx: fast reset
[   55.866403] Atheros AG71xx: update pll 2
[   55.870462] Atheros AG71xx: ath79_set_pllval: regmap: 0x81548000, pll_reg: 0x2c, pll_val: 0x02000000

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit 2a00054618)

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

@@ -127,6 +127,10 @@
 	};
 };
 
+&ref {
+	clock-frequency = <25000000>;
+};
+
 &spi {
 	status = "okay";