timekeeping: Use syscore_ops instead of sysdev class and sysdev

The timekeeping subsystem uses a sysdev class and a sysdev for
executing timekeeping_suspend() after interrupts have been turned off
on the boot CPU (during system suspend) and for executing
timekeeping_resume() before turning on interrupts on the boot CPU
(during system resume).  However, since both of these functions
ignore their arguments, the entire mechanism may be replaced with a
struct syscore_ops object which is simpler.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>