Exact audio DMA timings

[alyosha-tas]

Follow up from issue #13. Need to test things such as:

precise DMA timing, is there a delay after timer overflow? does writing to fifo control reset the fifo even if power is off? does writing to fifo with power off add samples to the fifo? what are the actual trigger conditions for DMA?

[alyosha-tas]

Findings so far:

fifo cannot be written to with power off (power off means FIFO is off.) DMA happens 4 cycles after overflow.