libfdt: More thorough use of constification

As a read-only functions, which take a const pointer to the fdt, treat
fdt_get_property() and fdt_getprop() as returning const pointers to
within the blob.  fdt_get_property_w() and fdt_getprop_w() versions
are supplied which take a non-const fdt pointer and return a non-const
pointer for the benefit of callers wishing to alter the device tree
contents.

Likewise the lower-level fdt_offset_ptr() and _fdt_offset_ptr()
functions are changed to return const pointers, with *_w() versions
supplied.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
diff --git a/tests/setprop_inplace.c b/tests/setprop_inplace.c
index bcfd357..59c1209 100644
--- a/tests/setprop_inplace.c
+++ b/tests/setprop_inplace.c
@@ -33,8 +33,9 @@
 int main(int argc, char *argv[])
 {
 	void *fdt;
-	uint32_t *intp;
-	char *strp, *xstr;
+	const uint32_t *intp;
+	const char *strp;
+	char *xstr;
 	int xlen, i;
 	int err;