Update MKR RGB lib to enable use on the Portenta H7. (A3->A5)

[jwestmoreland]

Requires jumper from A3 to A5.

First discussed here:.

[jwestmoreland]

Usage:

If using the Portenta H7 - uncomment:

// #define USING_PORTENTA_H7

in ..\libraries\Arduino_MKRRGB\src\MKRRGBMatrix.h (30).

use the library as before according to the existing examples.

[jwestmoreland]

This PR needs this to work: 955a927

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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