I was taking a look at the list and I've found something that may help you. Go to the link https://lists.zephyrproject.org/g/users/message/371 and check the solution. I think that maybe you can try something similar to that. Remember that the hardware used in the example is a nrf52 and yours is a nrf51, but the device names for this case are the same.
The STM32F103 I don't know the dev name. You can check that directly on the source code (board.h -> https://github.com/zephyrproject-rtos/zephyr/blob/d6e3f22fddccf4b542cb5ec780440cb629c12290/boards/arm/nucleo_f103rb/board.h). Maybe the name for the device is PORTA or PORTC it depends on the which one is the I2C port for your project.
I hope it helps.
Best regards, Rodrigo Peixoto. Ayna Tech +55 (82) 98144-8585.