The current code allocating the DART has a couple of bugs, first
it's called on all machines including the ones who have no DART
(oops), and then it tries to access the device-tree using the
"of_chosen" pointer before it was initialized.

The enclosed patch fixes these.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>