drivers: serial: bt: Add UART over NUS Bluetooth driver

Enables usage Bluetooth LE GATT as a serial endpoint to exchange data
using UART APIs. This implementation is compatible with UART Interrupt
Driven APIs and uses the nus-uart device-tree node properties to
configure FIFO buffers for transmitting and receiving. Defining
multiple instances of the driver is possible and it allows implementing
multiple GATT NUS service instances to exchange data as separate serial
endpoints.

Signed-off-by: Luis Ubieda <luisf@croxel.com>