mips: Add MIPSfpga platform support

MIPSfpga is an FPGA based dev platform.

In a nutshell, its a microAptiv cpu core with lots of Xilinx IP blocks

The FPGA dev board used is the Nexys4DDR board by Digilent.

For more information, check the Readme file in board/imgtec/xilfpga

Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Reviewed-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>