ARM: meson: document meson8 compatible properties

Add device tree bindings documentation for Amlogic Meson8 SoCs.

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