target/ppc: cpu_init: Move check_pow and QOM macros to a header

These will need to be accessed from other files once we move the CPUs
code to separate files.

The check_pow_hid0 and check_pow_hid0_74xx are too specific to be
moved to a header so I'll deal with them later when splitting this
code between the multiple CPU families.

Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20220216162426.1885923-27-farosas@linux.ibm.com>
Signed-off-by: Cédric Le Goater <clg@kaod.org>