dtc: Enable and fix -Wpointer-arith warnings

This patch turns on the -Wpointer-arith option in the dtc Makefile,
and fixes the resulting warnings due to using (void *) in pointer
arithmetic.  While convenient, pointer arithmetic on void * is not
portable, so it's better that we avoid it, particularly in libfdt.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/testutils.c

@@ -200,7 +200,7 @@ void save_blob(const char *filename, void *fdt)
 	int fd;
 	int totalsize;
 	int offset;
-	void *p;
+	char *p;
 	int ret;
 
 	fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0666);