mmc: documentation of mmc non-blocking request usage and design.

Documentation about the background and the design of mmc non-blocking.
Host driver guidelines to minimize request preparation overhead.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>