Merge patch "riscv: errata: improve T-Head CMO"

This fixes an encoding issue with T-Head's dcache.cva and fixes the
comment about the T-Head encodings.  The first of these was a fix and
got picked up earlier, I'm merging the second on top of it as they touch
the same comment.

* b4-shazam-merge:
  riscv: errata: prefix T-Head mnemonics with th.
  riscv: errata: fix T-Head dcache.cva encoding

Link: https://lore.kernel.org/r/20230827090813.1353-1-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>