dts: vt8500: Add initial dts support for WM8850

This patch adds a soc dtsi for the Wondermedia WM8850.

A board dts file is also included for the W70v2 tablet, with support
for all the drivers currently in mainline.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Olof Johansson <olof@lixom.net>