Update dtc/libfdt submodule to v1.4.7

dtc v1.4.7 contains a bunch of improvements to make libfdt safer against
handling a corrupted or malicious tree, which is a good thing to have.  It
also includes an explicit fdt checking function that we'll be wanting in
future.

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

diff --git a/dtc b/dtc
index e543880..88f1890 160000 (submodule)
--- a/dtc
+++ b/dtc
@@ -1 +1 @@
-Subproject commit e54388015af1fb4bf04d0bca99caba1074d9cc42
+Subproject commit 88f18909db731a627456f26d779445f84e449536