mmc: fix DT binding documentation SDHCI left-over

The file Documentation/devicetree/bindings/mmc/mmc.txt is common for all
MMC host drivers. Use a generic MMC host reference instead of an SDHCI
left-over.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>