rockchip: dts: rk3399-firely: add 'same-as-spl'

Like on rk3399-puma we want to continue booting the fill U-Boot from
the same device as the SPL stage.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>