powerpc/platforms/83xx/suspend: Remove write-only global variable

pmc_dev is only assigned in .probe(), otherwise the variable is unused.
So drop this pointer that serves no purpose.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220707061441.193869-3-u.kleine-koenig@pengutronix.de