device: dynamic device handles were declared as const

When CONFIG_HAS_DYNAMIC_DEVICE_HANDLES is selected, devices handles are
placed in RAM region so that they can be modified at runtime. However,
the gen_handles.py script added the `const` attribute to the device
handle arrays regardless of this setting, leading to faults when running
the application. This may be an indicator that this feature is not
actively used.

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