staging: unisys: add visorbus driver

This base driver provides bus functionality to visorhid, visorhba,
and visornic which will be later added to our driver base. Visorbus
supports sPar bus model and manages bus specific functionality. It
maintains the sysfs subtree /sys/devices/visorbus*/.It is
responsible for device creation and destruction of the devices on
its bus.

Signed-off-by: Erik Arfvidson <erik.arfvidson@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>