arm/dt: Basic versatile devicetree support

This patch adds adds very basic support for booting versatile with a
device tree.  It simply allows the existing machine_descs to match
against the versatile ab & pb compatible values so that the kernel can
boot.  Kernel parameters and the initrd pointer is read out of the
tree instead of atags.

This is not complete device tree support.  This change will be
reverted when a new machine_desc is added that can populate the
versatile device registrations directly from data in the tree instead
of using hard coded data.  That change will be made in a future patch.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>