PCI: qcom: Implement .assert_perst()

PCI: qcom: Implement .assert_perst()

Add support for assert_perst() for switches like TC9563, which require
configuration before the PCIe link is established. Such devices use this
function op to assert PERST# before configuring the device and once the
configuration is done they de-assert PERST#.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20251101-tc9563-v9-5-de3429f7787a@oss.qualcomm.com