riscv: add ISA extension parsing for Ztso

Add support to parse the Ztso string in the riscv,isa string. The
bindings already supports it but not the ISA parsing code.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Link: https://lore.kernel.org/r/20231220155723.684081-2-cleger@rivosinc.com


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