arm64: dts: allwinner: h6: Add Orange Pi 3 DTS

Orange Pi 3 is a H6 based SBC made by Xulong, released in January 2019. It
has the following features:

- Allwinner H6 quad-core 64-bit ARM Cortex-A53
- GPU Mali-T720
- 1GB or 2GB LPDDR3 RAM
- AXP805 PMIC
- AP6256 Wifi/BT 5.0
- USB 2.0 host port (A)
- USB 2.0 micro usb, OTG
- USB 3.0 Host + 4 port USB hub (GL3510)
- Gigabit Ethernet (Realtek RTL8211E phy)
- HDMI 2.0 port
- soldered eMMC (optional)
- 3x LED (one is on the bottom)
- microphone
- audio jack
- PCIe

Add basic support for the board.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>