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/P2020RDB-PC_defconfig b/configs/P2020RDB-PC_defconfig
index 4f3be9e..2afef4a 100644
--- a/configs/P2020RDB-PC_defconfig
+++ b/configs/P2020RDB-PC_defconfig
@@ -10,6 +10,7 @@
 # CONFIG_CMD_ERRATA is not set
 CONFIG_TARGET_P2020RDB=y
 CONFIG_MPC85XX_HAVE_RESET_VECTOR=y
+CONFIG_L2_CACHE=y
 CONFIG_ENABLE_36BIT_PHYS=y
 CONFIG_SYS_MPC85XX_NO_RESETVEC=y
 CONFIG_USE_UBOOTPATH=y