SubdeviceBackend still relies on fixed int32_t raw data type and uses raw access for target accessors

[mhier]

The SubdeviceBackend obtains the accessors of the target device with the AccessMode::raw flag and a fixed int32_t data type. Instead, no raw flag should be specified and the data type should depend on raw type of the subdevice registers (i.e. ratio of the number of bytes and of the number of words from the subdevice map file).

It should not be necessary to use raw access for the accessors of the target device. Doing so will restrict the usage of the subdevice backed to target devices which support raw access.