uart_dma example could be improved

[peterharperuk]

The uart_dma example is in the pio folder. But it can be used to test the uart periperal

1. There's a bug that the GPIO_FUNC_UART must be set before calling uart_init or you'll see some junk.
2. The example is quite complicated. It might be worth removing the uart and putting a version in the uart folder that demonstrates using dma with the uart?
3. Add a uart irq as it's "interesting" how this is impacted by dma. If uart fifos are enabled you won't see any uart irqs as dma is too quick to remove data from the uart fifo.
4. The example allows you to disable dma but this doesn't work for pio, as the fifos overflow? Might be worth using async workers to make it more realistic?