PCI: imx6: Assert PERST# before enabling regulators

The PCIe endpoint may start responding or driving signals as soon as its
supply is enabled, even before the reference clock is stable.  Asserting
PERST# before enabling the regulator ensures that the endpoint remains in
reset throughout the entire power-up sequence, until both power and refclk
are known to be stable and link initialization can safely begin.

Currently, the driver enables the vpcie3v3aux regulator in imx_pcie_probe()
before PERST# is asserted in imx_pcie_host_init(), which may cause PCIe
endpoint undefined behavior during early power-up. However, there is no
issue so far because PERST# is requested as GPIOD_OUT_HIGH in
imx_pcie_probe(), which guarantees that PERST# is asserted before enabling
the vpcie3v3aux regulator.

This prepares for an upcoming changes that will parse the reset property
using the new Root Port binding, which will use GPIOD_ASIS when requesting
the reset GPIO. With GPIOD_ASIS, the GPIO state is not guaranteed, so
explicit sequencing is required.

Fix the power sequencing by:

  1. Moving vpcie3v3aux regulator enable from probe to
     imx_pcie_host_init(), where it can be properly sequenced with PERST#.

  2. Moving imx_pcie_assert_perst() before regulator and clock enable to
     ensure correct ordering.

Signed-off-by: Sherry Sun <sherry.sun@nxp.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Richard Zhu <hongxing.zhu@nxp.com>
Link: https://patch.msgid.link/20260422093549.407022-4-sherry.sun@nxp.com

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