rockchip: spl: rk3399: implement chip-specific board_spl_was_booted_from()

To support the new "same-as-spl" specifier in the boot-order on the
RK3399, this implements the chip-specific mapping from the information
obtainable from the BROM to a OF path name.

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