usb: xhci: move xhci.h to include usb

The xhci.h header file is currently located under drivers/usb/xhci
Move it to the include/usb folder to make it available to drivers that
are not under drivers/usb/xhci

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/usb/host/xhci-mem.c b/drivers/usb/host/xhci-mem.c
index 84c2c33..530e979 100644
--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -21,7 +21,7 @@
 #include <asm/cache.h>
 #include <linux/errno.h>
 
-#include "xhci.h"
+#include <usb/xhci.h>
 
 #define CACHELINE_SIZE		CONFIG_SYS_CACHELINE_SIZE
 /**