[POWERPC] Improve resource setup of PowerMac G5 HT bridge

The device node for the HT bridge on G5s doesn't contain useful ranges.

We used to give it a bunch of the known PCI space and then punch a "hole"
in it based on where the AGP or PCIe region was.  This reworks it to
use the actual register in the bridge that controls the decoding instead.

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