Code is AVR specific but .properties claims support for all architectures

The .properties file indicates the the library works for all architectures.
```
architectures=*
```
but the code uses and depends on AVR specific port i/o.
The .properties should be updated to reflect that the library is only for the AVR core.
It would also be helpful to potential users and customers of the RGB-Matrix Panel to put some information about the library only working for AVR based arduinos in the README.txt file as well as on the actual product web pages on the adafruit site.

Inspect the library's .properties file and README.txt, then review the AVR-specific port I/O usage described in the issue. Done means the architecture declaration and README accurately state AVR-based Arduino support; product-page updates are outside this repository unless their location is provided.

Written by the indexing model from the issue text.