mmc: dt: Deduplicate binding docs by referencing mmc.txt

Now that we have common bindings for MMC, rewrite the individual
bindings to inherit from mmc.txt and describe their differences.

Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>