ARM: dts: add dtsi for Amlogic Meson8 SoCs

This adds a dtsi for Amlogic Meson8 SoCs. It differs from the Meson6
dtsi for the number of Cortex-A9 cores (4 vs 2) and for the frequency
of clk81.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Carlo Caione <carlo@caione.org>