[POWERPC] Stop using ppc_sys for Xilinx Virtex boards

The arch/ppc/syslib/ppc_sys.c infrastructure does not work well for the
virtex ports.  Move the ml300 and ml403 board ports over to use the new
virtex_devices infrastructure.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Paul Mackerras <paulus@samba.org>