[POWERPC] Rework Kconfig dependancies for Xilinx Virtex ppc405 platform

Reverse dependency order for Xilinx Virtex parts.  For these parts, It
makes more sense for boards/chips to specify which features they
provide instead of the features listing the parts they are implemented
in.  I think it also makes adding new board ports simpler.

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