powerpc: Add coherent_dma_mask to mv64x60 devices

DMA ops requires that coherent_dma_mask be set properly for a device,
but this was not being done for devices on the MV64x60 that use DMA.
Both the serial and ethernet devices need this or they won't be able
to allocate memory.

Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>