PCI: qcom: Add support for parsing the new Root Port binding

The DT binding has moved the PHY, PERST# properties to Root Port node from
the Host Bridge node. So add support for parsing the new binding. The new
binding uses 'reset-gpios' property for PERST#, hence parse the same
property in the driver instead of the legacy 'perst-gpios'.

To maintain DT backwards compatibility, fallback to the legacy method of
parsing the host bridge node if the properties are not present in the Root
Port node.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
[mani: refactored the root port parsing code, fixed a bug & commit message rewording]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250702-perst-v5-2-920b3d1f6ee1@qti.qualcomm.com