powerpc/4xx: Add support for ISA holes on 4xx PCI/X/E

This adds support for ISA memory holes on the PCI, PCI-X and
PCI-E busses of the 4xx platforms. The patch includes changes
to the Bamboo and Canyonlands device-trees to add such a hole,
others can be updated separately.

The ISA memory hole is an additional outbound window configured
in the bridge to generate PCI cycles in the low memory addresses,
thus allowing to access things such as the hard-decoded VGA
aperture at 0xa0000..0xbffff or other similar things. It's made
accessible to userspace via the new legacy_mem file in sysfs for
which support was added by a previous patch.

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