arm64: dts: rockchip: Add devicetree for NanoPC-T4

This adds a device tree for the NanoPC-T4 SBC, which is based on the
Rockchip RK3399 SoC and marketed by FriendlyELEC.

Known working:

- Serial
- Ethernet
- HDMI
- USB 2.0

All of the interesting stuff is in a .dtsi because there are at least
two other boards that share most of it: NanoPi M4 and NanoPi NEO4.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[rm: various further cleanup]
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>