bcm2835: consistent signal names.

[Gadgetoid]

I guess these are... weird... because either:

1. The datasheet done it, who are we to argue with peripheral signal nomenclature?
2. pinctl is now a critical part of some infrastructure and changing signal names will cause nuclear meltdown
3. It's too much like hard work to rename things now, it's all indented so pretty and such

On the off chance it's no 3, here's a PR to rename things like SDA0 to I2C0_SDA so there aren't two alt mode pin naming schemes coexisting, ending up in my board documentation files, and making my website look ugly.

IMHO things like SAn and SDn could stand to be prefixed with SMI and if I were being really pedantic (and assuming I'm not horribly wrong) things like ARM_TRST changed to JTAG_TRST.

Is this petty and unnecessary? Yes.

Could I sleep if I didn't raise this PR? No.

[pelwell]

The names have all been derived from the respective datasheets, with a degree of automated regularisation. Of those potential reasons for maintaining the status quo, it's 2 that worries me the most.

I hope that writing this PR has the same effect as writing a letter but not sending it, because without further encouragement I'm not planning on merging it.