usb: host: xhci: add platform driver support

This adds a fairly simple xhci-platform driver support. Currently it is
used by the dwc3 driver for supporting host mode.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
diff --git a/drivers/usb/host/xhci.h b/drivers/usb/host/xhci.h
index 57cd632..91074fd 100644
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1663,6 +1663,17 @@
 static inline void xhci_unregister_pci(void) {}
 #endif
 
+#if defined(CONFIG_USB_XHCI_PLATFORM) \
+	|| defined(CONFIG_USB_XHCI_PLATFORM_MODULE)
+int xhci_register_plat(void);
+void xhci_unregister_plat(void);
+#else
+static inline int xhci_register_plat(void)
+{ return 0; }
+static inline void xhci_unregister_plat(void)
+{  }
+#endif
+
 /* xHCI host controller glue */
 typedef void (*xhci_get_quirks_t)(struct device *, struct xhci_hcd *);
 void xhci_quiesce(struct xhci_hcd *xhci);