PCI: mvebu: Make Marvell PCIe driver depend on OF

The Marvell PCIe host controller driver is heavily tied to Device Tree
APIs, and can only be used on platforms where the Device Tree is
used.  Therefore, it should "depends on OF" to avoid build failures on
!OF configurations.

Reported-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>