The Rockchip ATF platform can be entered from both Coreboot and U-Boot.
While Coreboot does submit the list of linked parameter structs as
platform param, upstream u-boot actually always provides a pointer
to a devicetree as parameter.
This results in current ATF not running at all when started from U-Boot.

To fix this, add a stub that checks if the parameter is a fdt so we
can at least boot and not get stuck. Later on we can extend this with
actual parsing of information from the devicetree.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Cc: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>