docs: driver-model: Add documentation for sync_state

The sync_state() driver callback was added recently, but the
documentation was missing.  Adding it now.

Signed-off-by: Saravana Kannan <saravanak@google.com>
Link: https://lore.kernel.org/r/20191011191521.179614-4-saravanak@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>