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