m68k: Hook up rtc-generic

m68k has been a long time user of the generic RTC abstraction, so hook up
rtc-generic:
  - Create the "rtc-generic" platform device if mach_hwclk is set,
  - Add checks for mach_hwclk, in anticipation of RTC chip drivers being moved
    to drivers/rtc/.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Kyle McMartin <kyle@mcmartin.ca>