openfirm: Add OF_device_unregister_xref

When a device fails to attach, or a module is unloaded, we want to be
able to unregister the xref as the device goes away. While the device_t
may still be valid, it won't be providing whatever functionality the
consumer that follows the xref wants it for and thus, we should not
keep it discoverable.

[Commit message re-worded by kevans@]

Reviewed by: ian, imp
Differential Revision: https://reviews.freebsd.org/D22945