sparc64: refactor code to obtain cfg_handle property from MDESC

Refactors code to get the cfg_handle property of a node from 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>