PCI: mvebu: Setup PCIe controller to Root Complex mode

This driver operates only in Root Complex mode, so ensure that hardware is
properly configured in Root Complex mode.

Link: https://lore.kernel.org/r/20211125124605.25915-10-pali@kernel.org


Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>