PCI: mediatek-gen3: Add a .shutdown() callback to control PERST# signal

PCI: mediatek-gen3: Add a .shutdown() callback to control PERST# signal

Add a .shutdown() callback to control the timing of PERST# and power during
system shutdown to ensure that PERST# is asserted before power to the
connector is removed, as required by PCIe CEM r6.0, sec 2.2.

Signed-off-by: Jian Yang <jian.yang@mediatek.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20260413071401.1151-3-jian.yang@mediatek.com