litedram: Add support for Microwatt-initialized controller

litedram: Add support for Microwatt-initialized controller

This adds support for initializing the memory controller from microwatt
rather than using a built-in RiscV processor. This might require some
fixes to LiteX and LiteDRAM (they haven't been merged as of this commit
yet).

This is enabled in the shipped generated files and can be changed via
modifying the generator script to pass False to "mw_init"

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>