ARM: dts: rockchip: add dts for Firefly Firefly-RK3288 boards

This adds support for Firefly-RK3288, Rockchip RK3288 based development
boards made by Firefly.

There are 2 dts for 2 versions of the board. rk3288-firefly-beta.dts is
for the beta version, rk3288-firefly.dts is for the mass production version.

Signed-off-by: FUKAUMI Naoki <naobsd@gmail.com>
[removed gmac reference, due to the gmac node going through the net-tree]
Tested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>