nrf5: Adding rtc.c which implements the machine rtc module to be included in build.
nrf5: Adding rtc.c which implements the machine rtc module to be included in build.

adafruit/micropython,adafruit/micropython,adafruit/circuitpython,tralamazza/micropython,adafruit/circuitpython,tralamazza/micropython,adafruit/circuitpython,tralamazza/micropython,tralamazza/micropython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython