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: Leonard Crestez <leonard.crestez@nxp.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>