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>