riscv: ax25: Andes specific cache shall only support in M-mode

Limit the cache configuration only can be supported in M mode.
It can not be manipulated in S mode.

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