target/ppc: cpu_init: Move SPR registration macros to a header

Put the SPR registration macros in a header that is accessible outside
of cpu_init.c. The following patches will move CPU-specific code to
separate files and will need to access it.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-26-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>