drivers: serial: pl011: 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 pl011_regs *) are no
longer needed. PL011_REGS 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>