[POWERPC] Fix up ibm_architecture_vec definition

This problem was noticed by one of the Phyp firmware folks.
Our ibm,client-architecture-support call was failing.
This corrects the vector length parameters being passed in.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>