boards: shields: Add nRF9160 DK overlay for arduino_uno_click shield

The original Arduino Uno provides the same SCL/SDA on two sets of pins,
but the nRF9160 DK maps these pins to two different pairs of GPIO. When
using the Arduino Uno Click Shield board with the nRF9160 DK, the
P0.18/P0.19 pair must be used.

Also, the default pin group for the nRF9160 DK includes RTS/CTS HW flow
control, but the Arduino Uno Click Shield board does not connect these
pins (only TX/RX are connected on the shield). This keeps RX/TX on the
same pins, but just removes RTS/CTS from the pin groups.

References:

 - Arduino Uno R3 pinout:
   https://docs.arduino.cc/resources/pinouts/A000066-full-pinout.pdf

 - Arduino Uno R3 schematic:
   https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

 - Arduino UNO click shield schematic:
   https://download.mikroe.com/documents/add-on-boards/click-shields/arduino-uno/arduino-uno-click-shield-schematic-v101.pdf



Signed-off-by: Chris Wilson <chris@cgnd.dev>