Replace timer unit with RISC-V CLINT

[ezelioli]

This PR replaces the timer unit module with an OBI-compliant implementation of the RISC-V Core Local Interrupt Controller (CLINT). The idea is to have a cleaner, more understandable implementation and a RISC-V compliant software stack to manage interrupts. In addition, a configurable number of simpler OBI timer units (e.g. adapted from PULP's APB timer) can be added in parallel and connected to custom external interrupt lines. Finally, this is intended to simplify the documentation of interrupt handling in Croc.

Checklist:

• Add OBI CLINT RTL
• Add CLINT software driver
• Adapt helloworld test
• Remove timer_unit
• Add simple OBI timer(s)
• Add documentation (deferred to https://vlsi.ethz.ch/wiki/Interrupts)