nrf: port: save rtc value across reboots
nrf: port: save rtc value across reboots

As part of the reset process, save the current tick count to an
uninitialized memory location.  That way, the current tick value will be
preserved across reboots.

A reboot will cause us to lose a certain number of ticks, depending on
how long a reboot takes, however if reboots are infrequent then this
will not be a large amount of time lost.

Signed-off-by: Sean Cross <d7e19930cc1f42c2d0781f4d9e6f1fe5891bf9cf@xobs.io>

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