PCI: designware-ep: Configure Resizable BAR cap to advertise the smallest size

Configure the Resizable BAR capability to advertise the smallest size
(1MB) for a couple of reasons:

- Host side resource allocation of BAR fails for larger sizes
- Endpoint function driver does not allocate memory for all supported
  sizes in the Resizable BAR capability.

If and when there is a usecase required to add more flexibility using
the Resizable BAR capability, this can be revisited.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>