arm64: dts: mediatek: mt8395-genio-1200-evk: Move common parts to dtsi

In preparation for introducing the Genio 1200 EVK UFS board support, split
mt8395-genio-1200-evk.dts file in two to create mt8395-genio-common.dtsi
file, containing common definitions for both boards.

Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>