dt-bindings: PCI: dwc: rockchip: Add missing legacy-interrupt-controller

dt-bindings: PCI: dwc: rockchip: Add missing legacy-interrupt-controller

Rockchip RK356x and RK3588 handle legacy interrupts via a ganged
interrupts. The RK356x DT implements this via a sub-node named
"legacy-interrupt-controller", just like a couple of other PCIe
implementations. This adds proper documentation for this and updates
the example to avoid regressions.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20230731165723.53069-5-sebastian.reichel@collabora.com
Signed-off-by: Rob Herring <robh@kernel.org>