PCI: Add legacy_io/mem to all busses

Currently, only PHBs get the legacy_* files, which makes it tricky for
userland to get access to the legacy space.  This commit exposes them in
every bus, since even child buses may forward legacy cycles if
configured properly.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>