xen/tmem: do not allow XEN_TMEM on ARM64

tmem is not supported on arm or arm64 yet. Will revert this
once the Xen hypervisor supports it.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/Kconfig

@@ -145,7 +145,7 @@ config SWIOTLB_XEN
 
 config XEN_TMEM
 	tristate
-	depends on !ARM
+	depends on !ARM && !ARM64
 	default m if (CLEANCACHE || FRONTSWAP)
 	help
 	  Shim to interface in-kernel Transcendent Memory hooks