Acording to the description in `gpio_irq_hal_api.h` file `gpio_irq_init()` should return 0 on success.
Currently, it returns 1 causing the FPGA test to fail.
The 15.0 SDK doesn't support the nRF51 so it must continue to use the legacy
NRF drivers. Thus the original common_rtc.c, gpio_api.c and us_ticker.c are
restored and placed under TARGET_NRF5x/TARGET_NRF51.
Likewise the modified common_rtc.c, gpio_api.c and us_ticker.c are moved to
TARGET_NRF5x/TARGET_NRF52 so they are excluded from nRF51 builds.