greybus: endo: hook up endos into the device tree

This hooks up the endo, and modules, into the device tree.  All modules
for a specific endo are created when the host device is initialized.
When an interface is registered, the correct module for it is found and
that module is used for the sysfs tree.  When the interface is removed,
the reference on the module is dropped.

When the host device goes away, the whole endo and modules are removed
at once.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>