spl: Allow cache drivers to be used in SPL

Add an option for building cache drivers in SPL.

Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>