ARM64: dts: amlogic: Add basic support for Amlogic S905D

This patch introduces the basic support for the Amlogic S905D (MesonGXL)
and for the Amlogic evaluation boards P230 and P231.
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>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>