ARM64: dts: meson-gxl: add support for the S905W SoC and the P281 board

S905W is a new SoC from the GXL series. It is a cost-reduced version of
the S905X.
The P281 development board from Amlogic uses the same layout as the P231
(S905D development board). Thus the new P281 board inherits
meson-gx-p23x-q20x.dtsi to avoid code-duplication.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>