Add Support for CPU DMA

[bangcorrupt]

Add peripheral driver for CPU DMA and update existing peripheral drivers with DMA support.

[bangcorrupt]

@vananasun #38 (comment)

After studying EDMA3 and embedded topics a lot I definitely agree that EDMA3 transfers should be integrated into the EMIFA driver. It would be useful to sketch out together somewhere what the EDMA3 driver implementation should and shouldn't be able to do. To sum up use cases:

EDMA3 as a non-blocking memcpy
EDMA3 to (de-)interleave audio buffers
EDMA3 integrated into all peripheral drivers
EDMA3 ping pong buffer (supports LCD and other devices)
EDMA3 to end world hunger

If we start with a peripheral driver for the EDMA supporting 1D transfers with fixed or incrementing addresses, we can start profiling where it actually improves performance.

Servicing peripherals makes sense where it is a multi-byte protocol, like the panel MCU UART or LCD SPI, we can use the DMA to grab each buffer and only interrupt the CPU once we have a full message.

For single-byte protocols like MIDI, DMA won't reduce the frequency of interrupts and may only add overhead. We may be able to implement a ring buffer using linked DMA.

It's probably easiest to start with memory to memory transfers, as we don't have to integrate it with any peripheral handling. I've started work on a minimal bootloader with just support for serial console and debug LED, so we can develop individual drivers in isolation.