powerpc: implement arch_setup_pdev_archdata

We have a long standing issues with platform devices not have a valid
dma_mask pointer.  This hasn't been an issue to date as no platform
device has tried to set its dma_mask value to a non-default value.

Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>