rockchip: spl: add documentation for spl_node_to_boot_device()

rockchip: spl: add documentation for spl_node_to_boot_device()

In the expectation that the spl-boot-order code will eventually
gain use outside of mach-rockchip: let's add documentation on the
spl_node_to_boot_device() function, which is likely to become a
publicly exported function.

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