fdt: Tell the FDT library where the device tree is

fdt: Tell the FDT library where the device tree is

This change adds a call to set_working_fdt_addr near the end of u-boot
initialization which tells the fdt command/library where the device tree is.
This makes it possible to use the fdt command to look at the active device tree
since otherwise there would be no way to know what address it was at to set
things up manually.

Signed-off-by: Gabe Black <gabeblack@google.com>
Signed-off-by: Simon Glass <sjg@chromium.org>