This patch adds support for Milkymist's System Controller core. The model
has the following features:
 - support for shutting down and restarting the board
 - provide two timers and GPIO
 - provide registers for system identification and reading the boards
   capabilities
Signed-off-by: NMichael Walle <michael@walle.cc>
Signed-off-by: NEdgar E. Iglesias <edgar.iglesias@gmail.com>