cmd_fdt: Print the control fdt in terms of virtual memory

If you want to inspect the control device tree using the fdt command,
the "fdt address -c" command previously unhelpfully printed the phys
memory address of the device tree. That address could not then be used
to set the fdt address for inspection. Changed the resulting print to
one that can be copied directly to the 'fdt address <addr>' command.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>