dt-bindings: mmc: Convert jz4740-mmc doc to YAML

Convert the jz4740-mmc.txt documentation to YAML.

The ingenic,jz4770-mmc compatible string was added in the process, with
a fallback to ingenic,jz4760-mmc.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Rob Herring <robh@kernel.org>