sunxi: add PineCube board

PineCube is an IP camera development kit released by Pine64.

It comes with the following compoents:

- A mainboard with Sochip S3 SoC, a 16MByte SPI Flash, AXP209 PMIC,
a power-only microUSB connector, a USB Type-A connector, a 10/100Mbps
Ethernet port and FPC connectors for camera and daughter board.
- An OV5640-based camera module which is connected to the parallel CSI
bus of the mainboard.
- A daughterboard with several buttons, a SD slot, some IR LEDs, a
microphone and a speaker connector.

As the device tree is synchronized in a previous commit, just add it to
Makefile, create a new MAINTAINER item and provide a defconfig.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>