soc: riscv: andes_v5: add Andes EXEC.IT option

Andes EXEC.IT (Execution on Instruction Table) is supported by Andes
toolchain only. Andes toolchain will replaces suitable 32-bit instructions
with the 16-bit "exec.it <INDEX>" in which <INDEX> points to a
corresponding 32-bit instruction in look up table.

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>