usb: xhci-pci: Add support for reset controllers

Some atypical users of xhci-pci might need to manually reset their xHCI
controller before starting the HCD setup. Check if a reset controller
device is available to the PCI bus and trigger a reset.

Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629161845.6021-6-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci.h

@@ -1770,6 +1770,8 @@ struct xhci_hcd {
 	/* optional clocks */
 	struct clk		*clk;
 	struct clk		*reg_clk;
+	/* optional reset controller */
+	struct reset_control *reset;
 	/* data structures */
 	struct xhci_device_context_array *dcbaa;
 	struct xhci_ring	*cmd_ring;