Add support for data byte select signals

[mbitsnbites]

Currently the sdram_dqmh and sdram_dqml signals are hard-wired to zero. I assume that these two signals can be used for implementing "byte select" during write operations.

For my application (a custom soft CPU with 8/16/32-bit word writing capabilities), byte select functionality in the SDRAM controller would simplify the interface significantly (otherwise I'd need to do a full 32-bit read-modify-write cycle to support writing of individual bytes, for instance).

Would it be hard to implement?

[emb4fun]

Please take a look here: https://github.com/nullobject/sdram-fpga/pull/9#issue-1046841802