Follow the standard nodename pattern "^mmc(@.*)?$" defined in
Documentation/devicetree/bindings/mmc/mmc-controller.yaml

Otherwise, after the dt-binding is converted to json-schema,
'make ARCH=arm dtbs_check' will show warnings like this:

  sdhc@5a000000: $nodename:0: 'sdhc@5a000000' does not match '^mmc(@.*)?$'
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>