target/ppc: cpu_init: Reuse init_proc_745 for the 755

The init_proc_755 function is identical to the 745 one except for the
755-specific registers. I think it is worth it to make them share
code.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-21-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>