[RISCV] Add CSRs defined in the recently ratified Sscofpmf extension

The "RISC-V Count Overflow and Mode-Based Filtering Extension" was
ratified at the end of last year though hasn't yet been integrated in
the main specification documents (see
<https://wiki.riscv.org/display/TECH/Recently+Ratified+Extensions>).

This commit adds the CSRs defined by this extension in
<https://github.com/riscv/riscv-count-overflow/releases/download/v0.5.2/Sscofpmf.pdf>.

Differential Revision: https://reviews.llvm.org/D117308