dts: bindings: gpio: Add STEMMA-QT connector

Add a definition of a STEMMA QT connector to which I2C
devices can be connected.

Since STEMMA-QT does not specify uses other than I2C,
this is just a formal definition.
(It may be helpful when using this connector as a GPIO,
 which is not defined in STEMMA-QT)

Maybe in usual need to define i2c alias such like as,

```
stemma_qt_i2c: &i2c0 { }
```

Signed-off-by: TOKITA Hiroshi <tokita.hiroshi@gmail.com>
Co-authored-by: Benjamin Cabé <kartben@gmail.com>