Add realtime clock management on [4]

[m-roberts]

0xC1 4 byte register: MISC__REAL_TIME_COUNTER
Keeps track of UNIX time, so long as the MCU has power.
If the MCU loses power this number resets to 1556668800: (GMT): Wednesday, 1 May 2019 00:00:00.

Ideally would be set by RPi whenever the Pi is synced with a timeserver.

Pi should take the MCU's value on start of service if it is not set to the default in the hub, although we might want to consider if it's possible to prefer a time server.