riscv: ax25: Hide the ax25-specific Kconfig option

riscv: ax25: Hide the ax25-specific Kconfig option

There is no need to expose RISCV_NDS to the Kconfig menu as it is
an ax25-specific option. Introduce a dedicated Kconfig option for
the cache ops of ax25 platform and use that to guard the cache ops.

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