binman: Automatically include a U-Boot .dtsi file

For boards that need U-Boot-specific additions to the device tree, it is
a minor annoyance to have to add these each time the tree is synced with
upstream.

Add a means to include a file (e.g. u-boot.dtsi) automatically into the .dts
file before it is compiled.

The file uses is the first one that exists in this list:

   arch/<arch>/dts/<board.dts>-u-boot.dtsi
   arch/<arch>/dts/<soc>-u-boot.dtsi
   arch/<arch>/dts/<cpu>-u-boot.dtsi
   arch/<arch>/dts/<vendor>-u-boot.dtsi
   arch/<arch>/dts/u-boot.dtsi

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>