PCI: amd-mdb: Assert PERST# on shutdown

PCI: amd-mdb: Assert PERST# on shutdown

Add a shutdown handler for the AMD MDB PCIe host controller that
asserts the PERST# signal via GPIO before the system powers off or
reboots. This ensures the connected PCIe endpoint is held in reset
during shutdown.

Signed-off-by: Sai Krishna Musham <sai.krishna.musham@amd.com>
[mani: removed conditional check since GPIO APIs return safely if desc is NULL]
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Link: https://patch.msgid.link/20260427121227.290604-1-sai.krishna.musham@amd.com