[POWERPC] Virtex: Add generic Xilinx Virtex board support

Adds support for generic Xilinx Virtex boards.  Any board which specifies
"xilinx,virtex" in the compatible property will make use of this board
support.

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