Add support for the rk3399 excavator main board.
This board works in a combination with the sapphire SOM.

This board have been sold as the rk3399 evaluation board for commercial customers.
You can get more info from below link:
http://opensource.rock-chips.com/wiki_Excavator_sapphire_boardSigned-off-by: Jacob Chen <jacob-chen@iotwrt.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>