ARM: tegra: allow passing cboot DTB to the kernel

Some users may wish to pass the cboot-supplied DTB to the booted kernel
rather than having U-Boot load the DTB itself. To allow this, expose the
address of the cboot-supplied DTB in environment variable $fdt_addr. At
least when using extlinux.conf, if the user doesn't explicitly specify
which DTB to pass to the kernel, U-Boot passes the DTB referred to by
this variable.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>