Makefile: Fix dependency for u-boot-with-dtb.bin

Makefile uses binman to produce u-boot-with-dtb.bin target. As its input it
takes DTB file and u-boot binary without DTB, which is stored in file
u-boot-nodtb.bin. So fix target dependency.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Behún <kabel@kernel.org>