of: remove arch/$(SRCARCH)/boot/dts from include search path for CPP

Having arch/$(SRCARCH)/boot/dts as an include search path is not
very useful these days because some architectures such as ARM64,
MIPS have no DT in this directory.  Instead, they have DT in vendor
sub-directories.

With some DT files in ARM and PowerPC fixed, we can now drop this
include search path.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Rob Herring <robh@kernel.org>