ARM: dts: sunxi: Add sunxi-itead-core-common.dtsi

Itead have a core module board that can be populated with either
an Allwinner A10 or A20 SoC. This patch creates a common dtsi
which these boards can use.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>