tests: drivers: counter: nrf_rtc: Move overlay to boards dir

Move overlay to boards directory.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
diff --git a/tests/drivers/counter/counter_nrf_rtc/fixed_top/nrf52840dk_nrf52840.overlay b/tests/drivers/counter/counter_nrf_rtc/fixed_top/boards/nrf52840dk_nrf52840.overlay
similarity index 100%
rename from tests/drivers/counter/counter_nrf_rtc/fixed_top/nrf52840dk_nrf52840.overlay
rename to tests/drivers/counter/counter_nrf_rtc/fixed_top/boards/nrf52840dk_nrf52840.overlay