kunit: Fix platform mock to enable calling driver->remove.

Set up the minimal links between device and driver structs
in of_fake_probe_platform that enable the driver core code
to successfully call driver->remove on the fake-probed driver.

Signed-off-by: Iurii Zaikin <yzaikin@google.com>
Change-Id: I9fa73166e7b139e705f871aa85482f56e2afd3dd