pci: pcie-rcar-gen4: Add Renesas R-Car Gen4 DW PCIe controller driver

pci: pcie-rcar-gen4: Add Renesas R-Car Gen4 DW PCIe controller driver

Add R-Car Gen4 PCIe controller support for host mode.

This controller is based on Synopsys DesignWare PCIe. However, this
particular controller has a number of vendor-specific registers, and as
such, requires initialization code, including PHY firmware loading.

The PHY firmware loading is implemented in an entirely generic manner,
by calling a firmware loading script, which the user can configure in
a way they require. This provides the user with flexibility of loading
the PCIe firmware from whichever storage device they need to load it
from.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>