Add TQFP-52 with exposed pad for AD6645
It is a footprint for AD6645. It is TQFP-52 with exposed pad 6.5mmx6.5mm. The footprint is composed of LQFP-52 footprint with 6.5x6.5 pad taken from LQFP-64. Added rounded pads.
Thanks for creating a pull request to contribute to the KiCad libraries! To speed up integration of your PR, please check the following items:
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Provide a URL to a datasheet for the footprint(s) you are contributing http://www.analog.com/media/en/package-pcb-resources/package/pkg_pdf/tqfp_edsv/sv_52_1.pdf -
An example screenshot image is very helpful 
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If there are matching symbol or 3D model pull requests, provide link(s) as appropriate TBD -
Check the output of the Travis automated check scripts - fix any errors as required
