arm64: dts: broadcom: bcm2712: fix RP1 endpoint PCI topology

arm64: dts: broadcom: bcm2712: fix RP1 endpoint PCI topology

The node describing the RP1 endpoint currently uses a specific name
('rp1_nexus') that does not correctly reflect the PCI topology.

Update the DT with the correct topology and use generic node names.

Additionally, since the driver dropped overlay support in favor of a
fully described DT, rename '...-ovl-rp1.dts' to '...-base.dtsi' for
inclusion in the board DTB, as it is no longer compiled as a
standalone DTB.

Signed-off-by: Andrea della Porta <andrea.porta@suse.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/827b12ba48bb47bc77a0f5e5617aea961c8bc6b5.1766077285.git.andrea.porta@suse.com
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>