fdt: Add board specific dts inclusion

Some architectures, for example Microblaze, doesn't need a
splitted device tree because every Microblaze hw design is
different. The individuell Microblaze device tree will be
auto generated by FPGA design tools and could be used directly
with dtc.

The auto generated dts for Microblaze can not processed by CPP.
Unfortunately that is the default procedure in U-Boot to merge
a splitted device tree (substitution of ARCH_CPU_DTS).

Microblaze will never use the ARCH_CPU_DTS substitution and we
introduce the new board specific substitution variable BOARD_DTS
that points into vendor/board/dts subdir with the file name
of CONFIG_DEFAULT_DEVICE_TREE. The common dts file in vendor/dts
subdir (defined by CONFIG_DEFAULT_DEVICE_TREE) contain a single
include line that can processed by CPP:

    /include/ BOARD_DTS

Signed-off-by: Stephan Linz <linz@li-pro.net>
Tested-by: Michal Simek <monstr@monstr.eu>
Acked-by: Simon Glass <sjg@chromium.org>