lm32: add Milkymist SoftUSB support

lm32: add Milkymist SoftUSB support

This patch adds support for Milkymist's SoftUSB core. This model differ
from the real hardware in its functionality. The real hardware consits of a
tiny freely programmable microcontroller which controls the USB ports. For
simplicity reasons, this model emulates only keyboard and mouse input
devices, eg. input events translates directly to the corresponding expected
messages.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>