drivers: serial: stellaris: drop usage of uart_device_config

Create a driver specific configuration structure, containing the
required fields only. Since the config struct can now store a pointer to
the UART structure, casts from address to (struct _uart*) are no longer
needed. UART_STRUCT has also been dropped in favor of using the config
pointer directly now that it is possible.

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