riscv: add ISA extension parsing for Zacas

Add parsing for Zacas ISA extension which was ratified recently in the
riscv-zacas manual.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Link: https://lore.kernel.org/r/20231220155723.684081-5-cleger@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>