PCI: host-generic: Add common helpers for parsing Root Port properties

PCI: host-generic: Add common helpers for parsing Root Port properties

Introduce generic helper functions to parse Root Port device tree nodes and
extract common properties like reset GPIOs. This allows multiple PCI host
controller drivers to share the same parsing logic.

Define struct pci_host_port to hold common Root Port properties (currently
only list of PERST# GPIO descriptors) and add pci_host_common_parse_ports()
to parse Root Port nodes from device tree.

Also add the 'ports' list to struct pci_host_bridge to better maintain
parsed Root Port information.

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>
Link: https://patch.msgid.link/20260422093549.407022-3-sherry.sun@nxp.com