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>
diff --git a/libfdt/fdt_ro.c b/libfdt/fdt_ro.c
index e55872f..2a6e905 100644
--- a/libfdt/fdt_ro.c
+++ b/libfdt/fdt_ro.c
@@ -408,10 +408,10 @@
 					     &phandle, sizeof(phandle));
 }
 
-int _stringlist_contains(const void *strlist, int listlen, const char *str)
+int _stringlist_contains(const char *strlist, int listlen, const char *str)
 {
 	int len = strlen(str);
-	const void *p;
+	const char *p;
 
 	while (listlen >= len) {
 		if (memcmp(str, strlist, len+1) == 0)