sparc64: enhance VIO device probing

- Allocate IRQs for VIO devices during probing.
- Allow clients to specify if IRQs would be allocated for a given
  VIO device.
- Cache the device handle of the root node of channel-devices sub-tree in
  Machine Description (MDESC).

Signed-off-by: Jagannathan Raman <jag.raman@oracle.com>
Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>