dt-bindings: imx6q-pcie: Add required property for i.MX6SX

i.MX6SX needs a PCI 'power-domains' entry, so add it to its required
properties section.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>

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

@@ -41,6 +41,7 @@ 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
 
 Additional required properties for imx7d-pcie:
 - power-domains: Must be set to a phandle pointing to PCIE_PHY power domain