dt-bindings: MIPS: Add entry for the YLM RetroMini

Add an entry to ingenic/devices.yaml for the JZ4725B-based
YLM "RetroMini" RS-90.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>