staging: vme: keep track of registered buses

This patch adds a list which keeps track of all registered VME
buses. This is required for adding refcounting later to bridge
modules, something that is not currently implemented.

This is based on the changes introduced by Emilio G. Cota in the
patch:

    https://lkml.org/lkml/2010/10/25/486

Signed-off-by: Manohar Vanga <manohar.vanga@cern.ch>
Acked-by: Martyn Welch <martyn.welch@ge.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>