PM / Blackfin: Use struct syscore_ops instead of sysdevs for PM

Convert some Blackfin architecture's code to using struct syscore_ops
objects for power management instead of sysdev classes and sysdevs.

This simplifies the code and reduces the kernel's memory footprint.
It also is necessary for removing sysdevs from the kernel entirely in
the future.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>