PCI: microchip: Add Microchip PolarFire PCIe controller driver

Add support for the Microchip PolarFire PCIe controller when configured in
host (Root Complex) mode.

[bhelgaas: wrap lines to fit in 80 columns, fix trivial style issues]
Link: https://lore.kernel.org/r/20210125162934.5335-4-daire.mcnamara@microchip.com


Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
[lorenzo.pieralisi@arm.com: minor comments tweak]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rob Herring <robh@kernel.org>