pmac-zilog: add platform driver

Add platform driver support to the pmac-zilog driver, for m68k macs.
Place the powermac-specific code inside #ifdef CONFIG_PPC_PMAC.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>