PCI: imx: Initial imx7d pm support

On imx7d the pcie-phy power domain is turned off in suspend and this can
make the system hang after resume when attempting any read from PCI.

Fix this by adding minimal suspend/resume code. This will prepare for
powering down on suspend and reset the block on resume.

Code is only for imx7d but a very similar sequence can be used for
other SOCs.

Original-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
[lorenzo.pieralisi@arm.com: commit log update]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>