instantiate registers for some SDRAM pins to force them into IOBs

My design was sometimes suffering from random bit errors that only occured
in some compiled bitstreams. If something seemingly completely unrelated
was changed, the next compiled bitstream would work just fine.

Looking at the IOB properties tab in the ISE's design summary, it was
visible that the registers for the sd_data and sd_bs pins were not placed
in IOBs, even though the corresponding option was enabled in the map
process properies. My guess is that the delay between the unregistered IOBs
and the slice registers varied between compilations, which explains why the
problem only occured sometimes and only for some bits.

The instantiations are not pretty, but I haven't gotten a bit error since
implementing this change.

The sdCe_o, sdRas_o, sdCas_o, sdWe_o, sdCke_o and sdAddr_o seem to reliably
get their registers placed into IOBs, so these are left alone by this
change.
their registers placed into IOBs

Signed-off-by: wrtlprnft <wrtlprnft@cssbook.de>