Changes requirement for "vsubsbs" instruction

Changes requirement for "vsubsbs" instruction, which has been supported
since ISA 2.03. (Please see section 5.9.1.2 of ISA 2.03)

Reported-by: Paul A. Clarke <pc@us.ibm.com>
Signed-off-by: Paul A. Clarke <pc@us.ibm.com>
Signed-off-by: Leonardo Bras <leonardo@linux.vnet.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
(cherry picked from commit fcfbc18d00b10335310c9665edd6e04f2d152be8)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>