drivers/soc/litex: add LiteX SoC Controller driver

This commit adds driver for the FPGA-based LiteX SoC
Controller from LiteX SoC builder.

Co-developed-by: Mateusz Holenko <mholenko@antmicro.com>
Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>
Signed-off-by: Pawel Czarnecki <pczarnecki@internships.antmicro.com>
Signed-off-by: Stafford Horne <shorne@gmail.com>