Undocumented registers

[macpijan]

We have found that numerous registers are undocumented, but used in the code (Hostboot). We have also found some cases where the documentation is not consistent with the code (e.g. states the register is read-only, while Hostboot writes there).
Part of the documentation problems (in terms of undocumented registers) we have faced can be found there: https://github.com/3mdeb/openpower-coreboot-docs/blob/main/devnotes/undocumented_registers.md