riscv: errata: prefix T-Head mnemonics with th.

T-Head now maintains some specification for their extended instructions
at [1], in which all instructions are prefixed "th.".

Follow this practice in the kernel comments.

Link: https://github.com/T-head-Semi/thead-extension-spec

 [1]
Signed-off-by: Icenowy Zheng <uwu@icenowy.me>
Reviewed-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>