ARM / Samsung: Use struct syscore_ops for "core" power management

Replace sysdev classes and struct sys_device objects used for "core"
power management by Samsung platforms with struct syscore_ops objects
that are simpler.

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

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>