PCI: j721e: Add TI J721E PCIe driver

Add support for PCIe controller in J721E SoC. The controller uses the
Cadence PCIe core programmed by pcie-cadence*.c. The PCIe controller
will work in both host mode and device mode.
Some of the features of the controller are:
  *) Supports both RC mode and EP mode
  *) Supports MSI and MSI-X support
  *) Supports upto GEN3 speed mode
  *) Supports SR-IOV capability
  *) Ability to route all transactions via SMMU (support will be added
     in a later patch).

Link: https://lore.kernel.org/r/20200722110317.4744-14-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>