tegra: fdt: Enable FDT support for Ventana

This switches Ventana over to use FDT for run-time config instead of
CONFIG options.

At present Ventana does not have its own device tree file - it just uses
the Seaboard one.

Signed-off-by: Tom Warren <twarren@nvidia.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>