dt-bindings: imx6q-pcie: Handle various PD configurations

dt-bindings: imx6q-pcie: Handle various PD configurations

The i.MX SoCs have various power domain configurations routed into
the PCIe IP. MX6SX is the only one which contains 2 domains and also
uses power-domain-names. MX6QDL do not use any domains. All the rest
uses one domain and does not use power-domain-names anymore.

Document all those configurations in the DT binding document.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20221211024859.672076-2-marex@denx.de
Signed-off-by: Rob Herring <robh@kernel.org>