openrisc: add support for LiteX

This adds support for a basic LiteX-based SoC with a mor1kx soft CPU.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>
[shorne: Merged in soc-cntl patch, removed CROSS_COMPILE, sort MAINT.]
Signed-off-by: Stafford Horne <shorne@gmail.com>