[POWERPC] Workaround Pegasos incorrect ISA "ranges"

The Pegasos firmware doesn't create a valid "ranges" property for the
ISA bridge, thus causing translation of ISA addresses and IO ports to
fail. This fixes it, thus re-enabling proper early serial console to
work on Pegasos.

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