riscv: treat undefined exception codes as reserved

Undefined exception codes currently lead to an out-of-bounds array
access. Prevent this by treating undefined exception codes as
"reserved".

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>