ARM: imx6: DHCOM i.MX6 PDK: config SPL to load U-Boot fitImage with mulitple DTs

Configure fitImage for U-Boot with a device tree for imx6 quad/dual
and duallite/solo. This enables to support the imx6 derivates
quad/dual/duallite/solo with a single binary.

Signed-off-by: Ludwig Zenz <lzenz@dh-electronics.com>
Reviewed-by: Marek Vasut <marex@denx.de>