xen: Port Xen grant table driver from mini-os

Make required updates to run on u-boot.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
diff --git a/include/xen.h b/include/xen.h
index 6fe30b8..686b714 100644
--- a/include/xen.h
+++ b/include/xen.h
@@ -8,7 +8,8 @@
 /**
  * xen_init() - Xen initialization
  *
- * Map Xen memory pages, initialize event handler and xenbus.
+ * Map Xen memory pages, initialize event handler and xenbus,
+ * setup the grant table.
  */
 void xen_init(void);