Document bindings for R-Car V4M (R8A779H0) PCIe endpoint module.

Link: https://lore.kernel.org/linux-pci/20240822064459.1133748-3-yoshihiro.shimoda.uh@renesas.comSigned-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>