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>