ARM: imx6: dh-imx6: Enable d-cache early in SPL

Enable d-cache early in SPL right after DRAM is started up.
This reduces U-Boot proper load time by 650ms when loaded
from SPI NOR.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Philip Oberfichtner <pro@denx.de>
diff --git a/configs/dh_imx6_defconfig b/configs/dh_imx6_defconfig
index 4b6776e..c7fb2c6 100644
--- a/configs/dh_imx6_defconfig
+++ b/configs/dh_imx6_defconfig
@@ -1,4 +1,5 @@
 CONFIG_ARM=y
+CONFIG_SPL_SYS_L2_PL310=y
 CONFIG_ARCH_MX6=y
 CONFIG_SYS_TEXT_BASE=0x17800000
 CONFIG_SYS_MALLOC_F_LEN=0x1000