pci: pci-uclass: Add support for Enhanced Allocation in Bridges

If Enhanced Allocation capability is present in bridges, use it
to read the fixed sub-ordinate bus number.

Signed-off-by: Suneel Garapati <sgarapati@marvell.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>