cpu: Add CPU metadata om the Python classes

cpu: Add CPU metadata om the Python classes

The configuration scripts currently hard-code the requirements of each
CPU. This is clearly not optimal as it makes writing new configuration
scripts painful and adding new CPU models requires existing scripts to
be updated. This patch adds the following class methods to the base
CPU and all relevant CPUs:

 * memory_mode -- Return a string describing the current memory mode
                  (invalid/atomic/timing).

 * require_caches -- Does the CPU model require caches?

 * support_take_over -- Does the CPU support CPU handover?