Add rock5b overlays for PCIe endpoint mode support.

If using the rock5b as an endpoint against a normal PC, only the
rk3588-rock-5b-pcie-ep.dtbo needs to be applied.

If using two rock5b:s, with one board as EP and the other board as RC,
rk3588-rock-5b-pcie-ep.dtbo and rk3588-rock-5b-pcie-srns.dtbo has to
be applied to the respective boards.
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Link: https://lore.kernel.org/r/20240607-rockchip-pcie-ep-v1-v5-13-0a042d6b0049@kernel.orgSigned-off-by: Heiko Stuebner <heiko@sntech.de>