rockchip: firefly: configs: remove config_spl_of_platdata

We should remove config_spl_of_platdata to build u-boot-spl-dtb.bin rather than u-boot-spl-nodtb.bin
since we use spl_back_to_brom.

I miss it because i forget to clean build-dir..

Signed-off-by: Jacob Chen <jacob-chen@iotwrt.com>
Acked-by: Simon Glass <sjg@chromium.org>