arm64: dts: add Pine64 support

arm64: dts: add Pine64 support

The Pine64 is a cost-efficient development board based on the
Allwinner A64 SoC.
There are three models: the basic version with Fast Ethernet and
512 MB of DRAM (Pine64) and two Pine64+ versions, which both
feature Gigabit Ethernet and additional connectors for touchscreens
and a camera. Or as my son put it: "Those are smaller and these are
missing." ;-)
The two Pine64+ models just differ in the amount of DRAM
(1GB vs. 2GB). Since U-Boot will figure out the right size for us and
patches the DT accordingly we just need to provide one DT for the
Pine64+.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
[Maxime: Removed the common DTSI and include directly the pine64 DTS]
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>