ledmatrix: Enable sleep pin

[JohnAZoidberg]

On first hw rev no change. On second hw rev with pogo pins it'll go to sleep when the sleep pin is low.

[JohnAZoidberg]

TODO: Detect if sleep pin is wired up, if not, just ignore it. So that we can support the first hardware revision.

[JohnAZoidberg]

Done!

@zachfeldman would you like to try to verify on your unit? Just to check if the firmware still works well on the old hardware revision.

[JohnAZoidberg]

Btw if you are adventurous you can try to solder up the sleep pin by yourself. Not easy, Harris destroyed one pin while trying to do so. But luckily we have multiple unused ones that we can use, if needed.

[JohnAZoidberg]

Works fine on both hw revs.