switchtec: Set DMA coherent mask

Switchtec hardware supports 64-bit DMA, so set the correct DMA mask.  This
allows the CMA to allocate larger buffers for memory windows.

Signed-off-by: Boris Glimcher <Boris.Glimcher@emc.com>
Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>