FROMLIST: PCI: Add new start_link() & stop_link function ops

First controller driver probes, enables link training and scans the
bus. When the PCI bridge is found, its child DT nodes will be scanned
and pwrctrl devices will be created if needed. By the time pwrctrl
driver probe gets called link training is already enabled by controller
driver.

Certain devices like TC956x which uses PCI pwrctl framework needs to
configure the device before PCI link is up.

As the controller driver already enables link training as part of
its probe, the moment device is powered on, controller and device
participates in the link training and link can come up immediately
and maynot have time to configure the device.

So we need to stop the link training by using stop_link() and enable
them back after device is configured by using start_link().

Link: https://lore.kernel.org/lkml/20250412-qps615_v4_1-v5-0-5b6a06132fec@oss.qualcomm.com/
Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>

Author: krishnachaitanya-linux

include/linux/pci.h

@@ -807,6 +807,8 @@ struct pci_ops {
 	void __iomem *(*map_bus)(struct pci_bus *bus, unsigned int devfn, int where);
 	int (*read)(struct pci_bus *bus, unsigned int devfn, int where, int size, u32 *val);
 	int (*write)(struct pci_bus *bus, unsigned int devfn, int where, int size, u32 val);
+	int (*start_link)(struct pci_bus *bus);
+	void (*stop_link)(struct pci_bus *bus);
 };
 
 /*