bcm283x: Follow-up on #440: Updated DMA documentation

[bezineb5]

Constants refactoring (fixed comment on last PR and replaced the "4" by a constant)
Added documentation about the DMA channel.

Thanks!

[codecov-io]

Codecov Report

Merging #441 into master will increase coverage by 0.00%.
The diff coverage is 25.92%.

@@           Coverage Diff           @@
##           master     #441   +/-   ##
=======================================
  Coverage   63.48%   63.49%           
=======================================
  Files         122      122           
  Lines       12554    12554           
=======================================
+ Hits         7970     7971    +1     
+ Misses       4361     4360    -1     
  Partials      223      223           

Impacted Files	Coverage Δ
host/bcm283x/dma.go	43.34% <25.92%> (ø)
devices/lepton/lepton.go	87.19% <0.00%> (-1.22%)	⬇️
host/sysfs/thermal_sensor.go	83.84% <0.00%> (+2.30%)	⬆️

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[maruel]

gohci

[maruel]

Thanks!