PCI: cadence: Add EndPoint Controller driver for Cadence PCIe controller

This patch adds support to the Cadence PCIe controller in endpoint mode.

Since pieces of source code are shared with the host driver (Root
Complex mode), we create a new directory under drivers/pci dedicated to
the Cadence PCIe controller. The common code is placed into
drivers/pci/cadence/pcie-cadence.c and used by both the host and
endpoint controller drivers.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>