device core: Add flag to autoremove device link on supplier unbind

When using the device links without the consumers or
suppliers maintaining pointers to these links, a flag can
help in autoremoving the links on supplier driver unbind.
We remove these links only when the supplier's link to its
consumers has gone in DL_STATE_SUPPLIER_UNBIND state.

Signed-off-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Suggested-by: Lukas Wunner <lukas@wunner.de>