dt-bindings: imx6q-pcie: Add multi-pd bindings for imx6sx

The PCIe and PCIE_PHY blocks are in different power domains on imx6sx
and this needs to be described using multi-pd bindings.

This was not required until now because the power-domain of the PCIe
block (DISPLAY) was always on.
Signed-off-by: NLeonard Crestez <leonard.crestez@nxp.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.txt

@@ -41,7 +41,9 @@ Optional properties:
 Additional required properties for imx6sx-pcie:
 - clock names: Must include the following additional entries:
 	- "pcie_inbound_axi"
-- power-domains: Must be set to a phandle pointing to the PCIE_PHY power domain
+- power-domains: Must be set to phandles pointing to the DISPLAY and
+  PCIE_PHY power domains
+- power-domain-names: Must be "pcie", "pcie_phy"
 
 Additional required properties for imx7d-pcie:
 - power-domains: Must be set to a phandle pointing to PCIE_PHY power domain