PM / MIPS: Convert i8259.c to using syscore_ops

The code in arch/mips/kernel/i8259.c still hasn't been converted to
using struct syscore_ops instead of a sysdev for resume and shutdown.
As a result, this code doesn't build any more after suspend, resume
and shutdown callbacks have been removed from struct sysdev_class.
Fix this problem by converting i8259.c to using syscore_ops.

Reported-and-tested-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Ralf Baechle <ralf@linux-mips.org>