dtb: Add arch-vt8500 board files to arch/arm/boot/dts/Makefile

Add board files (vt8500-bv07, wm8505-ref and wm8650-mid)
to allow 'make dtbs' on arch-vt8500.

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