rockchip: doc: show packet rk3036 uboot image

show how to packet rk3036 uboot image and boot from SD

Signed-off-by: Lin Huang <hl@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Series-to: u-boot
Series-version: 8
Series-cc: Lin Huang <hl@rock-chips.com>