When moving the definition of ppc4xx_edac_driver further down, the
forward declarations can just be dropped.

Do this to reduce needless line repetition.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220917232013.489931-1-u.kleine-koenig@pengutronix.de