target/ppc: Move common SPR functions out of cpu_init

Let's leave cpu_init with just generic CPU initialization and
QOM-related functions.

The rest of the SPR registration functions will be moved in the
following patches along with the code that uses them. These are only
the commonly used ones.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-28-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>