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/Kconfig b/Kconfig
index f24e4f0..b4de17a 100644
--- a/Kconfig
+++ b/Kconfig
@@ -175,6 +175,7 @@
 
 config XEN
 	bool "Select U-Boot be run as a bootloader for XEN Virtual Machine"
+	select SSCANF
 	help
 	  Enabling this option will make U-Boot be run as a bootloader
 	  for XEN [1] Virtual Machine.