[sam] Add SAMx7x DMA + DAC

[chris-durand]

• [x] DAC driver
• [x] DMA Driver
  • [x] Block transfer mode
  • [x] Linked list transfer mode
• [x] Update modm-devices
• [x] Examples
  • [x] Block transfer with DAC
  • [x] Linked list transfer with DAC
  • [x] Simple DAC example without DMA
• [x] Test examples in hardware
• [x] Documentation for DMA driver

[chris-durand]

Please wait with merging. I'll run a final hardware test in about 3 hours when I have access to an oscilloscope. Just to be sure I broke nothing in a refactoring. I could merge after the test.

[chris-durand]

It's still working, I'll merge.