ARM64: dts: amlogic: Add basic support for Amlogic S905X

This patch introduces the basic support for the Amlogic S905X (Meson
GXL) and for the Amlogic evaluation board P212.
No documentation has been released yet for this SoC, so for now only the
bare minimum has been added in the DT.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>