PCI: imx: Add multi-pd support

On some chips the PCIe and PCIE_PHY blocks are in separate power domains
which can be power-gated independently. The PCI driver needs to handle
this by keeping both domain active.

This is intended for imx6sx where PCIe is in DISPLAY and PCIE_PHY in
its own domain. Defining the DISPLAY domain requires a way for PCIe to
keep it active or it will break when displays are off.

The power-domains on imx6sx are meant to look like this:
	power-domains = <&pd_disp>, <&pd_pci>;
	power-domain-names = "pcie", "pcie_phy";

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: Ulf Hansson <ulf.hansson@linaro.org>