PCI: mobiveil: Add PCIe Gen4 RC driver for Layerscape SoCs

Add a PCI host controller driver for Layerscape SoCs integrating
the Mobiveil GPEX IP.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@nxp.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Minghuan Lian <Minghuan.Lian@nxp.com>
Reviewed-by: Andrew Murray <amurray@thegoodpenguin.co.uk>