[POWERPC] Generic time suspend/resume code

This removes the time suspend/restore code that was done through
a PMU notifier in arch/platforms/powermac/time.c.

Instead, introduce arch/powerpc/sysdev/timer.c which creates a sys
device and handles time of day suspend/resume through that.

This should probably be replaced by using the generic RTC framework
but for now it gets rid of the arcane powermac specific hack.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>