PCI: imx6: Parse 'reset-gpios' in Root Port nodes

The current DT binding for pci-imx6 specifies the 'reset-gpios' property
in the host bridge node. However, the PERST# signal logically belongs to
individual Root Ports rather than the host bridge itself. This becomes
important when supporting PCIe Key E connector and the PCI power control
framework for pci-imx6 driver, which requires properties to be specified
in Root Port nodes.

Parse 'reset-gpios' from Root Port nodes and the PCIe bridge nodes under
the Root Port using the common helper pci_host_common_parse_ports(), and
update the reset GPIO handling to use the parsed port list from
bridge->ports. To maintain DT backwards compatibility, fall back to the
legacy method of parsing the host bridge node if the reset property is not
present in the Root Port nodes.

Since now the reset GPIO is obtained with GPIOD_ASIS flag, it may be in
input mode, so use gpiod_direction_output() instead of
gpiod_set_value_cansleep() to ensure the reset GPIO is properly
configured as output before setting its value.

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/20260422093549.407022-5-sherry.sun@nxp.com

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