devicetree: bindings: Add vendor prefix for Merrii Technology Co., Ltd.

Merrii Technology Co., Ltd. is a Chinese ARM integration developer that
specializes in Allwinner SoC based designs.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
devicetree: bindings: Document supported Allwinner sunxi SoCs

This adds a list of supported Allwinner SoC bindings.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>