RISC-V: Add CSR encodings for all HPMCOUNTERS

Linux kernel can directly read these counters as the HPMCOUNTERS CSRs are
accessible in S-mode.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>