[POWERPC] Xilinx: Register AC97 Controller Reference with the platform bus

Add AC97 platform bus registration for Xilinx Virtex platforms.

Signed-off-by: Joachim Foerster <JOFT@gmx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>