Convert CONFIG_L2_CACHE to Kconfig

This converts the following to Kconfig:
   CONFIG_L2_CACHE

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/MPC8548CDS_legacy_defconfig b/configs/MPC8548CDS_legacy_defconfig
index 34d1938..004175c 100644
--- a/configs/MPC8548CDS_legacy_defconfig
+++ b/configs/MPC8548CDS_legacy_defconfig
@@ -13,6 +13,7 @@
 # CONFIG_CMD_ERRATA is not set
 CONFIG_TARGET_MPC8548CDS=y
 CONFIG_MPC85XX_HAVE_RESET_VECTOR=y
+CONFIG_L2_CACHE=y
 CONFIG_ENABLE_36BIT_PHYS=y
 CONFIG_TARGET_MPC8548CDS_LEGACY=y
 CONFIG_USE_UBOOTPATH=y