target/ppc: cpu_init: Reuse init_proc_603 for the e300

init_proc_603 is defined after init_proc_e300, so I had to move some
code around to make it work.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-19-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>