PCI: rcar-gen4: Document how to obtain platform firmware

Renesas R-Car V4H (r8a779g0) has PCIe controller, and it requires
specific firmware downloading. So, add a document about the firmware
how to get.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
[kwilczynski: commit log, refactor the document content and then add
this new file to a correct index under the top-level PCI documentation]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250507100947.608875-1-yoshihiro.shimoda.uh@renesas.com