nrf: clock reimplementation breaks test

[pabigot]

tests/arch/arm/arm_irq_vector_table fails to build on nrf52_pca20020, nrf52_pca10040, and nrf52840_pca10056 starting at 6700f2f with this error:

make[3]: Leaving directory '/scratch/zephyr/zp1/zephyr/sanity-out/nrf52_pca20020/tests/arch/arm/arm_irq_vector_table/arch.interrupt'
make[3]: Entering directory '/scratch/zephyr/zp1/zephyr/sanity-out/nrf52_pca20020/tests/arch/arm/arm_irq_vector_table/arch.interrupt'
[ 98%] Building C object zephyr/CMakeFiles/zephyr_prebuilt.dir/misc/empty_file.c.obj
[100%] Linking C executable zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       25964 B       512 KB      4.95%
            SRAM:        5164 B        64 KB      7.88%
        IDT_LIST:          0 GB         2 KB      0.00/usr/local/zephyr-sdk-0.10.3/arm-zephyr-eabi/bin/../lib/gcc/arm-zephyr-eabi/8.3.0/../../../../arm-zephyr-eabi/bin/ld: ../app/libapp.a(arm_irq_vector_table.c.obj):/scratch/zephyr/zp1/zephyr/tests/arch/arm/arm_irq_vector_table/src/arm_irq_vector_table.c:167: undefined reference to `nrf_power_clock_isr'
collect2: error: ld returned 1 exit status
%
make[3]: *** [zephyr/CMakeFiles/zephyr_prebuilt.dir/build.make:101: zephyr/zephyr.elf] Error 1

This breaks CI checking of #19650.