xen: Port Xen bus driver from mini-os

Make required updates to run on u-boot and strip test code.

Signed-off-by: Anastasiia Lukianenko <anastasiia_lukianenko@epam.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
index 0ad35ed..9d0f604 100644
--- a/drivers/xen/Makefile
+++ b/drivers/xen/Makefile
@@ -4,3 +4,4 @@
 
 obj-y += hypervisor.o
 obj-y += events.o
+obj-y += xenbus.o