SDMA firmware

Epic: https://source.puri.sm/Librem5/use-cases/issues/110

The imx sdma engine can load updated firmware for dma transfers but it also has a ROM fallback

imx-sdma 302c0000.sdma: external firmware not found, using ROM firmware

There is also the possibility of writing custom sdma "scripts" so we could use open source firmware in the future.

http://billauer.co.il/blog/2011/10/imx-sdma-howto-assembler-linux/