script: dtc: clean generated files

Fix "make distclean" to clean up generated dtc files.

Without this patch the following files are left around:
 - dtc-lexer.lex.c
 - dtc-parser.tab.c
 - dtc-parser.tab.h
Signed-off-by: Magnus Damm <damm@opensource.se>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

scripts/dtc/Makefile

@@ -27,3 +27,5 @@ HOSTCFLAGS_dtc-parser.tab.o := $(HOSTCFLAGS_DTC)
 # dependencies on generated files need to be listed explicitly
 $(obj)/dtc-lexer.lex.o: $(obj)/dtc-parser.tab.h
 
+# generated files need to be cleaned explicitly
+clean-files	:= dtc-lexer.lex.c dtc-parser.tab.c dtc-parser.tab.h