CVA6 do not support cvxif dualwrite/dualread

[ZElkacimi]

CVA6 do not support dualread and dualwrite from a Coprocessor.

Also, related to dualwrite: It is mentioned in the cvxif spec, that we is 2 bits wide when XLEN = 32 and X_RFW_WIDTH = 64, and 1 bit wide otherwise. If we is 2 bits wide, then we[1] is only allowed to be 1 if we[0] is 1 as well (i.e. for dual writeback). The picture below, from cvxif spec, shows how it should be declared:

While in cva6, at cvxif_pkg.sv, the we signal is declared as follow:

We can notice two issues here, the fact that we is not declared correctly, and that, definitely, cva6 currently do not support dualwrite from a coprocessor.