xen: Add dependency on armv8

U-Boot currently implements hypervisor calls only for ARM64 that's why add
dependency on ARM64. The hardware functionality is also available on
ARMv7a, but currently not supported by U-Boot, hence it is not added to the
list of dependencies here.

Signed-off-by: Michal Simek <michal.simek@amd.com>
diff --git a/Kconfig b/Kconfig
index b4de17a..888b998 100644
--- a/Kconfig
+++ b/Kconfig
@@ -175,6 +175,7 @@
 
 config XEN
 	bool "Select U-Boot be run as a bootloader for XEN Virtual Machine"
+	depends on ARM64
 	select SSCANF
 	help
 	  Enabling this option will make U-Boot be run as a bootloader