RISC-V: Improve SBI definitions

Fixed few typos and bit fields not aligned with the spec. Define other
related macros that will be useful in the future.

Signed-off-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20220711174632.4186047-6-atishp@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>