Fix cp_uimm in cg_csritype

Created by: silabs-robin

As per https://github.com/openhwgroup/core-v-verif/issues/664 this PR updates cp_uimm in sampling of cg_csritype.

The problem was a slightly confusing term in riscv formats, uimm ("unsigned immediate") which name is used only in csr instructions and some compressed instructions. It is not the same as how imm differs between the instruction formats (e.g. U-type, R-type, etc). Hence, the existing immu[31:12] did not sample the immediate of csr instructions.

ps. There is also a question of whether "special" fields like uimm and shamt should be made into new properties of the instruction class or if it is fine to reuse the existing canonical fields like rs1 and rs2. (i vote for the latter)