Document platform-specific SW requirements

We need a document targeting people porting an operating system to OpenTitan (or writing baremetal software from scratch) where we outline all platform-specific requirements.

- Link to ibex documentation for information which CSR are available.
- Trap handlers which we expect to be there, e.g. https://github.com/lowRISC/opentitan/issues/78

This could be part of the earl grey toplevel docs (https://docs.opentitan.org/hw/top_earlgrey/doc/).

Start with the Earl Grey toplevel documentation location and the Ibex documentation linked in the issue. Review issue 78 for the expected trap-handler context, then define the platform-specific requirements for OS ports and bare-metal software. Done means a document covers the requirements and links the relevant CSR and trap-handler information.

Written by the indexing model from the issue text.