[feat] Add a sleep or wait function

[hgruniaux]

It is useful to be able to wait for some time or cycles in the kernel (e.g. because the hardware need time to do an operation).

Not to be confused with the sleep system call required for POSIX compatibility. The sleep system call let the scheduler do something else. The kernel sleep just wait some time so the hardware can catch up (in the kernel there is no scheduler for now).

Tutorial: https://github.com/bztsrc/raspi3-tutorial/tree/master/07_delays
The tutorial is not up-to date, QEMU now support BCM System Timer peripheral.