ARM64: dts: rockchip: add dts for RK3368 Sheep board

ARM64: dts: rockchip: add dts for RK3368 Sheep board

The Rockchip rk3368 top board maybe match to several types of screen,
such as LVDS, MIPI and EDP. It's confuse with current dts file.

This patch to add a dtsi file - rk3368-tb.dtsi, put the common nodes
and set their status to be "okay". But for the screen type, it hasn't
enable any type of screen, dts file should do it.

For example, the patch adds rk3368-tp-sheep.dts for Sheep board, which
uses mipi screen with rk818 pmic.

Change-Id: Ide5952552283a078f4eb7368507a7cf553275021
Signed-off-by: Jianqun xu <jay.xu@rock-chips.com>