rockchip: dts: rk3399-puma: set spl-payload-offset

defines the spl-payload to 256k (0x40000)

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>