dtoc: Rename the auto-generated dt-structs.h file

The filename of the auto-generated file is the same as the file that
includes it. Even though the form is in the generated/ subdirectory, this
could be confused.

Rename the generated file to something that makes it clear it is
auto-generated.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>