dt-bindings: memory-controllers: ingenic: Split out child node properties

Binding schemas which define child node properties such as memory
controllers with timing properties need a separate schema which can be
referenced from child device schemas. This is necessary for
unevaluatedProperties checks to work properly.

Move the ingenic,nemc child properties to its own file and reference
from ingenic,nand.yaml which describes a child NAND controller.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220525210140.2489866-1-robh@kernel.org