Allow to use simple_sdramcon with just -Ot simple_sdramcon True cmdline switch

simple_sdramcon is a smaller and less performant SDRAM controller compared
to LASMIcon.
It is usefull if your design does not fit anymore in a small FPGA.

By using this instead of LASMIcon you approximately save:

- 493 LUT (8% of Spartan 6 SLX9)
- 242 Flip Flops (2% of Spartan 6 SLX9)
- 128 Dual Ported RAM (8% of Spartan 6 SLX9)