PCI: imx: Add workaround for e10728, IMX7d PCIe PLL failure

This implements the workound described in the NXP IMX7d erratum e10728.

Initial VCO oscillation may fail under corner conditions such as cold
temperature. It causes PCIe PLL to fail to lock in the initialization
phase, which results in the PCIe link failing to come up.

The workaround is to disable Duty-Cycle Corrector (DCC) calibration
after G_RST.

To do this it is necessary to gain access to the undocumented and
currently unused PCIe PHY register bank. A new device tree node of type
"fsl,imx7d-pcie-phy" is created for the PHY block and the existing PCIe
device uses a phandle named "fsl,imx7d-pcie-phy" to point to it.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
[lorenzo.pieralisi@arm.com: updated log string, commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>