periph: add a PeripheralInfo class for metadata.

[megng1]

As suggested in #11 (comment).

This PR proposes a PeripheralInfo class to gather the resources of a peripheral, with the aim of advertising them to the BSP generator. A ConfigDict class is also provided to collect constants.

[codecov]

Codecov Report

Merging #19 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #19   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         7    +1     
  Lines          675       693   +18     
  Branches       142       145    +3     
=========================================
+ Hits           675       693   +18     

Impacted Files	Coverage Δ
nmigen_soc/periph.py	100.00% <100.00%> (ø)

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[whitequark]

I'll need some time to dig into this--please ping me on IRC in a week or two if I don't review this quickly enough.

[megng1]

Updates, based on feedback from the July 6th IRC meeting:

• Remove the constant dictionary. Its implementation will be the subject of another PR.
• Use an event.Source corresponding to the IRQ line of the peripheral, instead of an event map describing local events. This gives the BSP generator a mean to recover both:
  • the IRQ number of the peripheral (by calling .index(periph_info.irq) on the interrupt controller event map)
  • the numbers of local events, in order to clear their pending bits. (by querying periph_info.irq.event_map)
• Also, use getter-only properties instead of attributes, for added safety.

[whitequark]

Looks good other than the one comment.