riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores

The PMU on T-Head C9xx cores is quite similar to the SSCOFPMF extension
but not completely identical, so this series adds a T-Head PMU errata
that handlen the differences.

* 'riscv-pmu' of ssh://gitolite.kernel.org/pub/scm/linux/kernel/git/palmer/linux:
  drivers/perf: riscv_pmu_sbi: add support for PMU variant on T-Head C9xx cores
  RISC-V: Cache SBI vendor values