Improve interrupt documentation

[mariodruiz]

• Improve interrupt documentation by including this Peter's post https://discuss.pynq.io/t/zynq-ps7-interrupts/198/2
• Return a warning when interrupts are used, but they are not associated to an interrupt controller, thus preventing the Overlay creation to fail.

This Fixes Xilinx/PYNQ-Dev#338

[codecov-commenter]

Codecov Report

Merging #1246 (61f7616) into image_v2.7 (2e317aa) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@              Coverage Diff               @@
##           image_v2.7    #1246      +/-   ##
==============================================
- Coverage       46.74%   46.72%   -0.02%     
==============================================
  Files              27       27              
  Lines            4895     4897       +2     
  Branches          818      818              
==============================================
  Hits             2288     2288              
- Misses           2550     2552       +2     
  Partials           57       57              

Impacted Files	Coverage Δ
pynq/pl_server/hwh_parser.py	17.64% <0.00%> (-0.14%)	⬇️

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[PeterOgden]

We support interrupts without a controller - just only attached to interrupt line 0. There's even a test for it.

[mariodruiz]

Ready for review

[jancumps]

Maybe include this improvement: https://discuss.pynq.io/t/zynq-ps7-interrupts/198/10?
It solves the issue that the example failed every other execution.

I'll create a pull request.

[mariodruiz]

@jancumps,

Would you like to contribute these changes? f so, please open submit a PR

[jancumps]

I just did, #1318