devices: constify statically initialized device pointers

It is frequent to find variable definitions like this:

```c
static const struct device *dev = DEVICE_DT_GET(...)
```

That is, module level variables that are statically initialized with a
device reference. Such value is, in most cases, never changed meaning
the variable can also be declared as const (immutable). This patch
constifies all such cases.

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