m68k/mac: Skip VIA port setup unless RTC is connected

Those Mac models which don't connect their RTC to VIA1 port B probably
have something else connected to those pins. Just leave them the way we
found them. Make the port B setup conditional on via_type, to match the
RTC accessors in arch/m68k/mac/misc.c.

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