m68k: Reinstate mac rtc

Reinstate the Mac hardware clock for CUDA ADB and Mac II ADB models.
It doesn't work properly on Mac IIsi ADB and PMU ADB yet, so leave them 
out.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>