devices: constify device pointers initialized at compile time

Many device pointers are initialized at compile and never changed. This
means that the device pointer can be constified (immutable).

Automated using:

```
perl -i -pe 's/const struct device \*(?!const)(.*)= DEVICE/const struct
device *const $1= DEVICE/g' **/*.c
```

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>