ARM: imx: vining2000: Enable fitImage support

The fitImage support was enabled in the downstream U-Boot port and the
kernel images on the device are fitImage, yet this functionality is not
enabled in mainline U-Boot. Enable it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Silvio Fricke <silvio.fricke@softing.com>
Cc: Stefano Babic <sbabic@denx.de>