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/truncated_property.c b/tests/truncated_property.c
index dbd3652..5642d8d 100644
--- a/tests/truncated_property.c
+++ b/tests/truncated_property.c
@@ -32,7 +32,7 @@
 int main(int argc, char *argv[])
 {
 	void *fdt = &_truncated_property;
-	void *prop;
+	const void *prop;
 	int err;
 	int len;