Pinout: Document PULP Cluster pins

Created by: Silabs-ArjanB

The Pulp Cluster functionality is not currently described in the User Manual. I added the core-v-docs/cores/cv32e40p/user_manual/source/pulp_cluster.rst file to start the documentation for this feature (pull request will be issued soon). I think that the description is not yet good enough to really know how to use clock_en_i and core_busy_o. For example, I would like to state something like the following: 'clock_en_i is allowed to be set to 0 when core_busy_o = 0'. I wonder if that is really the intention of this interface or not. The difficulty is in the reset/inital behavior of core_busy_o (it is 0 during reset and for a couple of cycles after reset); @davideschiavone can you please comment on how you expect that clock_en_i is used during reset plus the initial cycles of CV32E40P while core_busy_o is still 0? Davide, can you also please review the text I wrote so far (based on info from https://github.com/openhwgroup/cv32e40p/issues/338) and specifically provide some text for core_busy_o's description signal in the table and give your opinion on whether core_busy_o should be ignored when PULP_CLUSTER = 0 (as I wrote in my text).

In case core_busy_o is only useful for PULP_CLUSTER = 1, should we maybe rename clock_en_i and core_busy_o to pulp_clock_en_i and pulp_core_busy_o respectively to highlight that these signals are part of the PULP Cluster interface?