PCI: imx6: Wait for clocks to stabilize after ref_en

For boards without a reset GPIO we skip the delay between enabling the
pcie_ref_clk and touching the RC registers for configuration.  This hangs
the system if there isn't a proper delay to ensure the clocks are settled
in the DW PCIe core.

Also iMX6Q always needs an additional 10us delay to make sure the reset is
propagated through the core, as we don't have an explicitly controlled
reset input on this SoC.

This fixes a problem with 3fce0e88 ("PCI: imx6: Delay enabling
reference clock for SS until it stabilizes"): the kernel doesn't boot on
systems that don't pass the PCI GPIO reset in the DTB.  This regression
affects mx6 nitrogen boards.

[bhelgaas: add regression info in changelog]
Fixes: 3fce0e88 ("PCI: imx6: Delay enabling reference clock for SS until it stabilizes")
Reported-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Richard Zhu <richard.zhu@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>