Add cycle counter driver and test app

[hudson-ayers]

This PR adds a driver for the cycle counter capsule, for which kernel support is being added in https://github.com/tock/tock/pull/3934. It also adds a basic test application which prints the number of cycles required to toggle an LED, after checking that the ability to start and stop the driver has not already been claimed by another app.

Tested by adding the capsule to the nrf52840dk and running this app. It takes about 4000 cycles to toggle an LED from userspace.

[bradjc]

Can I ask you follow the new guide? https://github.com/tock/libtock-c/blob/43525b8fedff3531685dbb20b08916addcca0997/doc/guide.md

Maybe not the syscalls folder but the naming convention?

[hudson-ayers]

I will come back to this once the new libtock format is merged