samples: blinky: add support for non-standard led displays

[AndOrangutan]

This is a change to enable the blinky sample to work on matrix displays such as the common learner board MicroBit v2.

[github-actions]

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[sambhurst]

Hello @AndOrangutan, thanks for extending the blinky sample code so that it works on boards without a simple LED connection. Could you updated the README.rst to include the additional LED and how the two LED pins work together to toggle a single matrix LED. And that the additional LED is optional for non-matrix LED displays, but if it's implemented, blinky will simply toggle both LEDs.

[kartben]

This is complexifying blinky quite a bit for a sample that's meant to be pretty much the next step after "hello world" for most users. I might be missing somethign but it looks like what's needed is rather a new binding/driver that would allow to configure a given LED from an LED matrix as an LED device?