sunxi: add a defconfig for SoPine w/ official baseboard

The SoPine is a SoM by Pine64, with an Allwinner A64 SoC, a LPDDR3 DRAM
chip, an AXP803 PMIC, a SPI NOR Flash and a MicroSD slot. The card
detect pin of the MicroSD slot is broken, however, it doesn't matter as
the design of SoPine didn't allow hot-swapping the MicroSD card (The
MicroSD slot is at the back of the SoM, and when the SoM is installed on
the baseboard, it's nearly impossible to remove the MicroSD).

The official baseboard of it is a board with nearly the same connectors
with the original Pine64+, with the MicroUSB power jack replaced, and
at the position of MicroSD slot a eMMC module slot is added.

Add support for SoPine with the official baseboard by adding its
defconfig file. It still uses the device tree of Pine64, however, it
will change after a proper device tree of SoPine with baseboard is
accepted by Linux mainline.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[Update board/sunxi/MAINTAINERS]
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com>