ARM: meson: dts: add basic Meson/Meson6/Meson6-atv1200 DTSI/DTS

The Meson6 SoC is produced by Amlogic inc. and it is based on 2 Cortex
A9 and an ARM Mali-400 GPU.
This patch adds two basic DTSI for the preliminary support of Meson and
Meson6 SoCs. Another DTS is also added for supporting the atv1200 board,
produced by Geniatech inc.

Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>