xen: Fix Kconfig dependencies

XEN config can be enabled by other platforms (even it doesn't need to make
sense) that's why fix dependencies. XEN (xenbus.c) requires sscanf (also
pvblock needs it). And PVBLOCK is inside drivers/xen folder which requires
XEN to be enabled.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
index 0ee74d0..6cb9149 100644
--- a/drivers/xen/Kconfig
+++ b/drivers/xen/Kconfig
@@ -1,6 +1,6 @@
 config PVBLOCK
 	bool "Xen para-virtualized block device"
-	depends on DM
+	depends on DM && XEN
 	select BLK
 	help
 	  This driver implements the front-end of the Xen virtual