lm32: add Milkymist HPDMC support

lm32: add Milkymist HPDMC support

This patch adds support for the Milkymist's High Performance Dynamic Memory
Controller. This is just a dumb model without any functionality. While the
real hardware acts for example as a bridge between software and hardware
for sending SDRAM commans, this model will only eat up these commands and
always returns the expected hardware states, eg. PLL locked etc.

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