ARM: dts: add device tree for Mecer Xtreme Mini S6

The Mecer Xtreme Mini S6 features a Rockchip RK3229 SoC,
1GB DDR3 RAM, 8GB eMMC, MicroSD port, 10/100Mbps Ethernet,
Realtek 8723BS WLAN module, 2 x USB 2.0 ports, HDMI output,
and S/PDIF output.

Signed-off-by: Justin Swartz <justin.swartz@risingedge.co.za>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>