bcm2835_sdhci: formalize DMA tag/segment scaling requirements

This allows easy and care-free scaling of NUM_DMA_SEGS with proper-ish
calculations to make sure we can actually handle the number of segments we'd
like to handle on average so that performance comparisons can be easily made
at different values if/once we can actually handle it. It also makes it
helps the untrained reader understand more quickly the reasoning behind the
choice of maxsize/maxsegs/maxsegsize.