MIPS: CI20: Reduce clocksource to 750 kHz.

The original clock (3 MHz) is too fast for the clocksource,
there will be a chance that the system may get stuck.
Reported-by: Nikolaus Schaller <hns@goldelico.com>
Tested-by: Nikolaus Schaller <hns@goldelico.com> # on CI20
Signed-off-by: 周琰杰 (Zhou Yanjie) <zhouyanjie@wanyeetech.com>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/boot/dts/ingenic/ci20.dts

@@ -525,10 +525,10 @@ pins_mmc1: mmc1 {
 
 &tcu {
 	/*
-	 * 750 kHz for the system timer and 3 MHz for the clocksource,
+	 * 750 kHz for the system timer and clocksource,
 	 * use channel #0 for the system timer, #1 for the clocksource.
 	 */
 	assigned-clocks = <&tcu TCU_CLK_TIMER0>, <&tcu TCU_CLK_TIMER1>,
 					  <&tcu TCU_CLK_OST>;
-	assigned-clock-rates = <750000>, <3000000>, <3000000>;
+	assigned-clock-rates = <750000>, <750000>, <3000000>;
 };