[systeminfo] Support for CPU clock speed

[Novanic]

It would be very useful to get the current CPU clock speed of the local computer.

For example the famous Raspberry Pi reduces the clock speed when the CPU becomes hot, especially the new Raspberry Pi 5 which may require an active cooler. I would use that to display the current clock speed on a wall display to see if the clock speed is currently reduced and to optimize the clock speed and cooling behaviour (I'm planning to reduce the clock speed permanently to reduce fan starts).