greybus: Greybus driver: add a new callbacks to driver

Add connection_create and connection_destroy callbacks.

ES2 can map a cport to a pair of endpoints.
Because ES2 have only a few pair of endpoints, ES2 need to have
access to some high level connection information such as protocol id
to effectively map the cports.
These callback will provide these information and help ES2 to map cports.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>