mips: ingenic: Enable EXT/2 divider on JZ4750/55/60 if EXT is 24 MHz

The JZ4750, JZ4755 and JZ4760 (non-B version) support using a 24 MHz
external crystal oscillator instead of the typical 12 MHz one.

However, most of the SoC's IP blocks only work with a 12 MHz clock.
Thanksfully, there is a /2 divider we can enable when a 24 MHz external
crystal is present.

Force-enable this /2 divider when the oscillator is 24 MHz, so that the
SoC always uses a 12 MHz clock internally.

It is done here, and not in the clocks driver, because we need the EXT
clock to be 12 MHz for the early console to work, and the clocks driver
probes way too late.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>