More SAMx7x ADC features

[chris-durand]

• [x] Oversampling
• [x] Multi-channel conversion sequences
• [x] Hardware triggering
• [x] Examples
  • [x] Multi-channel sequence
  • [x] DMA with hardware timer trigger
• [x] Test examples in hardware

[salkinium]

@chris-durand Please rebase and merge when you have time.

[chris-durand]

I'll merge it tomorrow.

[chris-durand]

I have pushed another commit to fix a driver bug.