arm64: dts: meson: add common SM1 ac2xx dtsi

Add a common dtsi for Android STB devices based on the Amlogic S905X3
(AC213/AC214) and S905D3 (AC201/AC202) reference designs. The dtsi is
loosely based on the existing SEI610 device-tree.

Popular SS905X3/D3 devices often ship in three configurations:

- 2GB/16GB with 10/100 internal PHY ethernet
- 4GB/32GB with 1GB external PHY ethernet
- 4GB/64GB with 1GB external PHY ethernet

Device dts filenames based on the common dtsi should include a 2g/4g
suffix, as this helps users to select a device-tree that matches the
Ethernet PHY spec for their device, e.g.

- meson-sm1-tvbox-2g.dts
- meson-sm1-tvbox-4g.dts

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>