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

Replace sysdev classes and struct sys_device objects used for "core"
power management by the PXA platform code with struct syscore_ops
objects that are simpler.

This 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>