mmc: mmci: Add required documentation for Device Tree bindings

This provides full documentation detailing each compulsory,
optional and device (variant) specific properties available
for use within the MMCI Device Tree.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Arnd Bergmann <arndb@arndb.de>
Signed-off-by: Chris Ball <cjb@laptop.org>