ARM: dts: renesas: Stop using the -u-boot DTs for build

The U-Boot build system can automatically paste -u-boot.dtsi at the
end of matching .dts during build. Stop emulating this behavior and
rename the -u-boot.dts files to -u-boot.dtsi, drop "#include...dts"
from those new u-boot.dtsi files, and update board configuration
accordingly.

The rename, '#include...dts` scrubbing and configuration update has
been done using the following script:
```
$ find . -name r[78]\*-u-boot.dts | sort -u | while read line ; do \
  git mv ${line%-u-boot.dts}-u-boot.dts ${line%-u-boot.dts}-u-boot.dtsi ; \
  done
$ sed -i '/^#include.*dts"/ d' `find . -name r[78]\*-u-boot.dtsi`
$ sed -i 's@-u-boot@@g' `git grep -li renesas configs`
```
The Salvator-X and ULCB board files have been updated manually.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Acked-by: Adam Ford <aford173@gmail.com>