doc: rockchip: add rk3066 Rikomagic MK808

Add rk3066 Rikomagic MK808 to the list of
mainline supported Rockchip boards.
Include instructions for creating and programming
images to NAND and SD card.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>