Enable PCI suspend/resume support on imx6sx SOCs. This is similar to
imx7d with a few differences:

* The PM_Turn_Off bit is exposed through an IOMUX GPR, like all other
pcie control bits on 6sx.
* The pcie_inbound_axi clk needs to be turned off in suspend. On resume
it is restored via resume -> deassert_core_reset -> enable_ref_clk.

Most of the resume logic is shared with the initial reset after probe.
Signed-off-by: NLeonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NAndrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: NLucas Stach <l.stach@pengutronix.de>