greybus: Documentation/sysfs: add a proposed sysfs tree for greybus

This adds a proposed sysfs layout for greybus to Documentation to make
it easier for people to discuss / test things.  It includes a module, an
interface, a bundle, and a gpbridge binding to that bundle.

This was discussed on the projectara software mailing list.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>