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>
diff --git a/drivers/Makefile b/drivers/Makefile
index cb8c215..bfc5a25 100644
--- a/drivers/Makefile
+++ b/drivers/Makefile
@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0+
 
+obj-$(CONFIG_$(SPL_TPL_)CACHE) += cache/
 obj-$(CONFIG_$(SPL_TPL_)CLK) += clk/
 obj-$(CONFIG_$(SPL_TPL_)DM) += core/
 obj-$(CONFIG_$(SPL_TPL_)DFU) += dfu/