doc: rockchip: Document Rockchip miniloader flashing

This would be useful and recommended boot flow for new boards
which has doesn't have the DDR support yet in mainline.

Sometimes it is very useful for debugging mainline DDR support.

Documen it for px30 boot flow.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever <Yang&lt;kever.yang@rock-chips.com>