Add a single-vendor config for vt8500. We can't enable WM8750 in
multi_v7_defconfig since it's a v6-based device, but it's still valuable
to have an in-tree defconfig that is suitable for the hardware.

This is based on multi_v7_defconfig and can be tweaked over time. It
gets us off the ground for now. Naming it vt8500_v6_v7 similar to i.MX
since there are v5-based vt8500 chips as well.
Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Tony Prisk <linux@prisktech.co.nz>