Convert the ingenic,jz4780-nemc.txt doc file to ingenic,nemc.yaml.

The ingenic,jz4725b-nemc compatible string was added in the process,
with a fallback to ingenic,jz4740-nemc.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Rob Herring <robh@kernel.org>