target/ppc: cpu_init: Move e300 SPR registration into a function

This is done to improve init_proc readability and to make subsequent
patches that touch this code a bit cleaner.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-17-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>