mmc: dt: Consolidate DT bindings

This patch unifies the current DT MMC bindings documentation and code,
adds generic MMC DT bindings documentation, and updates .dts files for
consistency.

[cjb: typo fixes, addition of max-frequency property]
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>