m68k/mac: Enable RTC for 100-series PowerBooks

According to Apple's Developer Notes, all of the early PowerBook models
have their RTC connected to VIA1. Use the VIA clock ops as appropriate.
This was tested on a PowerBook 170.

Don't use the VIA ops when not appropriate. Calling unimplemented clock
or PRAM getter or setter ops can now result in an error instead of
failing silently.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>