[POWERPC] Make U4 PCIe work on maple

The Maple support code was missing code for U4/CPC945 PCIe. This adds
it, enabling it to work on tigerwood boards, and possibly also js21
using SLOF. Also disable an obsolete firmware workaround.

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