drivers: serial: uart_miv: Disable SERIAL_SUPPORT_INTERRUPT

In the real MiV platform, UART does not have any interrupts routed to
the interrupt controller.

Up to this stage, the driver implemented a separate thread to simulate
interrupt operation to enable shell samples. This is not required anymore,
as Zephyr can run shell on interrupt-less UARTs.

Signed-off-by: Mateusz Holenko <mholenko@antmicro.com>