powerpc: Use of for_each_node_by_name() instead of open-coding it

Instead of manually coding the loop with of_find_node_by_name(), let's
switch to the standard macro for iterating over nodes with given name.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
[mpe: Fix build failures due to typo in mpc832x_mds.c]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>