[POWERPC] Fix Maple platform ISA bus

The Maple platform has ISA IOs but didn't call the new functions to
actually map those, thus crashing when trying to access the nvram.

This fixes Maple and JS2x using SLOF.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>