powerpc: Hook up rtc-generic, and kill rtc-ppc

PowerPC has been a long time user of the generic RTC abstraction, so hook up
rtc-generic:
  - Create the "rtc-generic" platform device if ppc_md.get_rtc_time is set,
  - Kill rtc-ppc, as rtc-generic offers the same functionality in a more
    generic way, and supports autoloading through udev.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>