litedram: Regenerate

litedram: Regenerate

Regenerate from upstream litex. Something in the update has improved
memory read and write performance quite a lot on my Nexys Video:

Before:

  Write speed: 83.2MiB/s
   Read speed: 140.4MiB/s

After:

  Write speed: 352.1MiB/s
   Read speed: 218.5MiB/s

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>