target-cris: Introduce CRISCPU subclasses

Use class_init functions to initialize the VR in preparation for
overriding v32+ behavior there.

Move cpu_cris_init() to cpu.c and hook up a class_by_name callback.

This change leads to unknown -cpu model names no longer falling back
to a CPU with VR 32 but instead returning NULL.

Signed-off-by: Andreas Färber <afaerber@suse.de>