arm64: dts: amlogic: add fbx8am DT overlays

Add support for two variants of the fbx8am board.

Signed-off-by: Pierre-Hugues Husson <phhusson@freebox.fr>
Signed-off-by: Marc Gonzalez <mgonzalez@freebox.fr>
Link: https://lore.kernel.org/r/79ba726d-d02c-44b9-b6f6-59b17ba9755c@freebox.fr
[narmstrong: squashed https://lore.kernel.org/all/814c9035-1fda-4cd3-ab3c-95b58e2ecb22@freebox.fr/

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Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>