Update dtc to fix compilation problem on Mac OS 10.6

Currently QEMU does not build on Mac OS 10.6
because of a missing patch in the dtc
subproject. Updating dtc to make the patch
available fixes this problem.
Signed-off-by: NJohn Arbuckle <programmingkidx@gmail.com>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

dtc @ e5438801

-Subproject commit 558cd81bdd432769b59bff01240c44f82cfb1a9d
+Subproject commit e54388015af1fb4bf04d0bca99caba1074d9cc42