powerpc/44x: Fix address decoding setup of PCI 2.x cells

The PCI 2.x cells used on some 44x SoCs only let us configure the decode
for the low 32-bit of the incoming PLB addresses. The top 4 bits (this
is a 36-bit bus) are hard wired to different values depending on the
specific SoC in use. Our code used to work "by accident" until I added
support for the ISA memory holes and while at it added more validity
checking of the addresses.

This patch should bring it back to working condition. It still relies
on the device-tree being correct but that's somewhat a pre-requisite
for anything to work anyway.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>