MIPS: ingenic: Add support for the JZ4725B SoC

Add preliminary support for boards based on the JZ4725B SoC from
Ingenic.

The JZ4725B SoC is supposed to be older than the JZ4740 SoC, but its
internals are much closer to what can be found on the JZ4750 and newer
SoCs.

It is low-power SoC with a MIPS32r1 SoC running at ~360 MHz, and no FPU.

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