Add a "parent" struct device to our PCI host bridge data structure so that
PCI can be rooted off another device in sysfs.

Note that arch/ppc doesn't use it, only arch/powerpc, though it's available
for both 32 and 64 bits.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>