arm64: allwinner: h6: add support for Pine H64 board

Pine H64 is an Allwinner H6-based SBC from Pine64, with the following
features:

- 1GiB/2GiB/4GiB LPDDR3 DRAM (in 4GiB situation only 3GiB is
accessible)
- AXP805 PMIC
- Raspberry-Pi-compatible GPIO header, "Euler" GPIO header (not
compatible with the "Euler" on Pine A64) and "Expansion" pin header
- 2 USB 2.0 ports and 1 USB 3.0 ports
- Audio jack
- MicroSD slot and eMMC module slot
- on-board SPI NOR flash
- 1Gbps Ethernet port (via RTL8211E PHY)
- HDMI port

Adds initial support for it, including the UART on the Expansion pin
header.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>