x86: Separate out U-Boot and device tree in ROM image

At present binman does not support updating a device tree that is part of
U-Boot (i.e u-boot.bin). Separate the entries into two so that we can get
updated entry information. This makes binman_entry_find() work correctly.

Do the same for SPL tool.

In both cases, group the two parts into a section so that SPL symbols get
the correct total size.

It may be possible for binman to handle this automatically at some point,
by ignoring u-boot.bin and always creating it from u-boot-nodtb.bin and
u-boot.dtb

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>