Improve address type clarity and semantic typos

[neilramaswamy]

Hi,

Thanks for this great repo on DMA! It was fun to read.

I've made a few changes to the README that should clarify areas that I had to read more than once to understand. Most changes here should be benign.

However, I updated the physical and bus offset addresses to match up with the datasheet; in section 1.2.3 on ARM physical addresses (page 6), the physical offset is 0x20000000 and the bus offset is 0x7E000000. I am a bit concerned about my change here because the code (dma-demo.c:49) does use 0x3F000000; it's unclear why this works. I don't see any reference to 0x3F000000 in the datasheet, so I'm unsure what the README should actually contain.

Thanks!