riscv: complete the list of exception codes

riscv: complete the list of exception codes

Only the first four exception codes are defined. Add the missing
exception codes from the definition in RISC-V Privileged Architecture
Version 1.10.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>