PCI: mobiveil: Add callback function for interrupt initialization

The Mobiveil GPEX internal MSI/INTx controller is not implemented
in all platforms in which the Mobiveil GPEX is integrated.

Allow platforms to implement their specific interrupt initialization.

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>