powerpc/cell/iommu: Improve error message for missing node

Some devices don't have a correct node ID and thus can't be
attached to an iommu.

The message displayed by the iommu code isn't very useful if
you don't have a device-tree node as it tries to print the
device-tree path but not the struct device name.

Improve this by printing the device name as well.

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