MMIO: Port the VideoCommon CP MMIOs to the new interface (and provide framework for other video related mappings).
MMIO: Port the VideoCommon PE MMIOs to the new interface.
MMIO: Port the SW CP/PE MMIOs to the new interface.

Migration is now complete.
Remove the old MMIO access "interface".
Use a pointer instead of a non-const ref in MMIO::Mapping::Read now that compatibility with the old interface is not required anymore.