[POWERPC] Don't define a custom bd_t for Xilixn Virtex based boards.

Why create a platform specific board_info structure that is hacked
together, ugly, and dangerous, when we've got a perfectly fine common
board_info structure that is hacked-together, ugly and dangerous.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Paul Mackerras <paulus@samba.org>