PCI: mobiveil: Introduce a new structure mobiveil_root_port

The Mobiveil PCIe controller can work in either Root Complex
mode or Endpoint mode.

Introduce a new structure mobiveil_root_port and abstract the
RC related members into it so that the code can be used by both
modes.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <amurray@thegoodpenguin.co.uk>