rockchip: Also load the initrd below 512M

Similar to load an fdt, when loading an initrd about the 512Mb mark
things seem to break. For now force loading below 512Mb until the reason
why this fails has been determined/solved.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Acked-by: Simon Glass <sjg@chromium.org>