configs/qemu: convert versatile to dt

As a followup to http://patchwork.ozlabs.org/patch/548550/

 fully convert
the versatile defconfig to create the dtb and update the instructions.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>