MIPS: ingenic: Add support for the RS90 board

The RS-90, better known as RetroMini, is a small and pocketable handheld
gaming console from YLMChina. It has little more than a JZ4725B SoC, a
NAND, a screen, some buttons and a speaker.

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