Given there's no public specification to this date, and in order
to capture some important details and singularities about the
controller let's document them once and for good.

Cc: linux-doc@vger.kernel.org
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>