[AVR32] Initialize dma_mask and dma_coherent_mask

The current at32ap7000 platform devices aren't declared as supporting DMA,
so that layered drivers can't tell whether they need to manage DMA.

This patch makes all those platform devices report that they support DMA.
Most do, but in a few cases this is inappropriate.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>