Documentation: start documenting driver design patterns

After realizing that we tend to tell developers the same thing over
and over, let's attempt to document some commin design patterns
used in the device drivers. The idea is that this can be extended
so I just start out with two well-known design patterns.

Cc: Rob Landley <rob@landley.net>
Cc: Mark Brown <broonie@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>