[PATCH] powerpc: kill ppc64 rtc.c, use genrtc instead

This moves the rtas RTC callbacks to rtas-rtc.c in arch/powerpc/kernel,
and kills the rest of arch/ppc64/kernel/rtc.c which was just a duplicate
of the genrtc functionality. Also enable build of genrtc for
CONFIG_PPC64 (it just works are we already have the required callbacks)
and enable it in all defconfigs.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>