SPARC64: VIO: Support for virtual-device MD node probing

SPARC64: VIO: Support for virtual-device MD node probing

This update adds support to the mdesc/vio infrastructure to
probe for "virtual-device" nodes in the MD. The vio
module will create sysfs device files for these nodes which
can be accessed by user space code (such as udev). In addition,
VIO drivers can now probe for these MD nodes if the need arises.

This functionality will serve as part of the fix for
BUG 24841906.

Signed-off-by: Aaron Young <Aaron.Young@oracle.com>
Reviewed-By: Alexandre Chartre <Alexandre.Chartre@oracle.com>
Orabug: 24841906