nvdimm: Add realize, unrealize callbacks to NVDIMMDevice class

nvdimm: Add realize, unrealize callbacks to NVDIMMDevice class

A new subclass inheriting NVDIMMDevice is going to be introduced in
subsequent patches. The new subclass uses the realize and unrealize
callbacks. Add them on NVDIMMClass to appropriately call them as part
of plug-unplug.

Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Acked-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Message-Id: <164396253158.109112.1926755104259023743.stgit@ltczzess4.aus.stglabs.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>