pcie: controller: add callback to get allocation base

When enumerating bridges, the allocated memory base and limit must be
programmed in the bridge config registers.

The base is programmed when starting enumerating the bridge and the
limit is programmed when all endpoints of the bridge and sub-bridges
are enumerated.

Add a second callback to get the base of the next allocation in
order to fill these base & limit values.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>