bcm283x: revamp DMA support

[maruel]

Ported from google/periph#172.

This is about cleaning up the RPi DMA code for all generations.

Also see #34 for RPi4 specific issue.

[maruel]

Also including issue google/periph#198 about misuse of dmaWaitcyclesMax.

[kirwinrMK]

DMA appears completely broken on RPI.
line 916 in bcm283x fails due to a miss match in units:
l := int(int64(d) * int64(w.Frequency()) / int64(physic.Hertz)) // Bits should be `l := int(d / w.Frequency().Period()) // Bits'

Once that's fixed, there's still a SIGBUS error coming from Nanospin, that could be because I'm using a CM4 though.