Figure out how to hook LiteDRAM to the WDDR PHY

[mithro]

@waviousllc recently released an DDR PHY under an Apache 2.0 license. The PHY is currently being used with proprietary memory controllers and it would be awesome to see it work with LiteDRAM instead.

As this is the first time LiteDRAM will be targeting a ASIC PHY rather an FPGA, it is likely there will be a bunch of experimentation needed to make things work, hence I'm creating this tracking issue.

[mithro]

See also https://github.com/waviousllc/wav-lpddr-hw/issues/12