arm64: dts: rockchip: Split out common nodes for Rock960 based boards

Since the same family members of Rock960 boards (Rock960 and Ficus)
share the same configuration, split out the common nodes into a common
dtsi file for reducing code duplication. The board specific nodes for
Ficus boards are then placed in corresponding board DTS file.

Below are some of the key differences between both Rock960 and Ficus
boards:

1. Different host enable GPIO for USB
2. Different power and reset GPIO for PCI-E
3. No Ethernet port on Rock960

Only the properties which differ between both boards are placed in the
board specific dts and the reset of the nodes are placed in common dtsi
file.
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>