andes: ae350: Implement cache switch via Kconfig

Kconfig provides SYS_[I|D]CACHE_OFF config options to switch off caches.
Provide the corresponding implementation to the options.

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Yu Chien Peter Lin <peterlin@andestech.com>