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.c b/libfdt/fdt.c
index 0d16427..6b2ca1f 100644
--- a/libfdt/fdt.c
+++ b/libfdt/fdt.c
@@ -76,7 +76,7 @@
 
 const void *fdt_offset_ptr(const void *fdt, int offset, int len)
 {
-	const void *p;
+	const char *p;
 
 	if (fdt_version(fdt) >= 0x11)
 		if (((offset + len) < offset)