Implementation of PCA9685

[mrusme]

Contribution description

Added new driver for PCA9685, which is commonly found on PWM breakouts.

Testing procedure

See tests/driver_pca9685. Simply connect a PCA9685 breakout to your board, set the correct I2C address and make it lighten up an LED attached to PWM & GND for example by calling pca9685_set_pwm(&pca9685, 0, 4096, 0, true, false);.

Info

Retry of #12033. @benpicco please check again!

[gschorcht]

@mrusme This PR is a duplicate of #PR 10556 which is waiting since December last year for merge. Please try to search for existing PRs before you start new developments. I invested a lot of time into the development of the PCA9685 driver. Even though PR #10556 can use the PWM extension API proposed in #PR 10533, it does not depend on it.

@benpicco I would suggest to close this PR since it is a duplicate and to review PR #10556.
Furthermore, there is an agreement that APIs of extenders should be as identical as possible to the API in periph/pwm.h That is, they have to implement exactly the same functions. This is not the case in this PR.

[mrusme]

@gschorcht thanks! However:

... which is waiting since December last year...

That's pretty much the point. Needed this now and there was nothing ready to use. Also, if a driver for a module the PCA9685 takes 9 months and still isn't ready to for use the iterative process might not been the best. Maybe try to do smaller steps, in order to make it available quicker and afterwards iterate on it to make it fancy. By that people (like me) could chip in and add features to it as well, when needed.

I can close this PR however, as I don't need it to be upstream. Subscribed to your PR and will follow the process there.