Improve RTC support

[ezio-melotti]

#332 added support for the PCF8523, however there are still a few things that can be improved:

• The PCF8523 should be added to sensors.toml, so that commands like sensors-info correctly detect it;
• The setup-rtc command should check if the PCF8523 is available before attempting the setup;
• A command to check time info using timedatectl status could be added too;
• A set-rpi-time command could be added too;
• The "software-based" solution implemented in Add driver for the pcf8523 RTC #320 could still be merged, even if it will be likely kept disabled;

#320 already adds some of these things, that could be cherry-picked into separate PRs.