riscv: ax25: cache: Add SPL_RISCV_MMODE for SPL

The mcache_ctl csr only can be manipulated in M mode.
Add SPL_RISCV_MMODE for U-Boot SPL to control cache
operation.

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: KC Lin <kclin@andestech.com>
Cc: Alan Kao <alankao@andestech.com>