FROMGIT: 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>
(cherry-pick from commit a3caeb8ffe5d2bbe01da66081f0ef28c26302d99
 https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/driver-core.git driver-core-next)
Conflicts: Documentation/driver-api/driver-model/driver.rst
[File was in a different location]
Bug: 142657042
Change-Id: I7ee9c61be93fb247759dbd6d90214c6994fe1bb0