Documentation: kbuild: Remove obsolete dtc_cpp section

Commit b40b25ff ("kbuild: always
run gcc -E on *.dts, remove cmd_dtc_cpp") improved the functionality of
cmd_dtc_cpp and merged it back into cmd_dtc.
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/kbuild/makefiles.txt

@@ -1163,29 +1163,6 @@ When kbuild executes, the following steps are followed (roughly):
 		clean-files += *.dtb
 		DTC_FLAGS ?= -p 1024
 
-    dtc_cpp
-	This is just like dtc as describe above, except that the C pre-
-	processor is invoked upon the .dtsp file before compiling the result
-	with dtc.
-
-	In order for build dependencies to work, all files compiled using
-	dtc_cpp must use the C pre-processor's #include functionality and not
-	dtc's /include/ functionality.
-
-	Using the C pre-processor allows use of #define to create named
-	constants. In turn, the #defines will typically appear in a header
-	file, which may be shared with regular C code. Since the dtc language
-	represents a data structure rather than code in C syntax, similar
-	restrictions are placed on a header file included by a device tree
-	file as for a header file included by an assembly language file.
-	In particular, the C pre-processor is passed -x assembler-with-cpp,
-	which sets macro __ASSEMBLY__. __DTS__ is also set. These allow header
-	files to restrict their content to that compatible with device tree
-	source.
-
-	A central rule exists to create $(obj)/%.dtb from $(src)/%.dtsp;
-	architecture Makefiles do no need to explicitly write out that rule.
-
 --- 6.8 Custom kbuild commands
 
 	When kbuild is executing with KBUILD_VERBOSE=0, then only a shorthand