spl: cache: Allow cache drivers in SPL

When ax25-ae350 try to enable v5l2 cache
driver in SPL configuration, it need this
option for cache support in SPL.

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