powerpc: Fix build of some FSL platforms

Commit 87ec0e98cfdd8b68da6a7f9e70142ffc0e404fbb in kumar's next branch
broke one of my test configs since it looks like Anton forgot about
that mpc832x_rdb platform which still uses the old style probing for
the SPI stuff.

I'll let them do a cleaner fix that probably involves changing the
probing method and getting rid of the platform device but for now
this will do to fix it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>