drivers: net: ot: Select UART_INTERRUPT_DRIVEN with UART RCP communication

The Openthread communication with RCP via UART (with HDLC protocol) is
performed with serial driver using interrupts. As it was tested with
echo_client sample program, the
CONFIG_SHELL_BACKEND_SERIAL_API_INTERRUPT_DRIVEN is defined by default for
it. This config selects aforementioned UART_INTERRUPT_DRIVEN.

Problem starts when somebody wants to integrated the driver as a standalone
one (without echo_client) as a part of user application.
In this situation the UART_INTERRUPT_DRIVEN is not defined by default and
there is no serial communication between RCP and HOST devices.

Signed-off-by: Lukasz Majewski <lukma@denx.de>