add iCE40LP8K-CM81 symbol

Created by: penoud

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Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:

• Provide a URL to a datasheet for the symbol(s) you are contributing http://www.latticesemi.com/~/media/LatticeSemi/Documents/DataSheets/iCE/iCE40LPHXFamilyDataSheet.pdf
• An example screenshot image is very helpful
• Ensure that the associated footprints match the official footprint library
• If there are matching footprint PRs, provide link(s) as appropriate https://github.com/KiCad/kicad-footprints/pull/936
• Check the output of the Travis automated check scripts - fix any errors as required