rockchip: rk3399: inject 'u-boot, spl-boot-device' for next-stage

rockchip: rk3399: inject 'u-boot, spl-boot-device' for next-stage

This implements the new 'spl_perform_fixups' hook for RK3399-based
boards and injects the /chosen/u-boot,spl-boot-device with an ofpath
corresponding to the boot device used.

The intended usage is for the full U-Boot stage to evaluate this in
scripts and then adapt its boot-order when using distro-boot.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>