PCI: layerscape: Add Freescale Layerscape PCIe driver

Add support for Freescale Layerscape PCIe controller.  This driver re-uses
the Synopsis DesignWare core code.

[bhelgaas: add Kconfig dependency on CONFIG_ARM]
Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>