powerpc/powernv: Fetch PHB bus range from dev-tree

The patch enables fetching bus range from device-tree for the
specific PHB. If we can't get that from device-tree, the default
range [0 255] will be used.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>