riscv: Add ISA extension parsing for Ziccrse

Add support to parse the Ziccrse string in the riscv,isa string.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrea Parri <parri.andrea@gmail.com>
Link: https://lore.kernel.org/r/20241103145153.105097-12-alexghiti@rivosinc.com


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