arm64/xen: introduce CONFIG_XEN and hypercall.S on ARM64

Introduce CONFIG_XEN and the implementation of hypercall.S (that is
the only ARMv8 specific code in Xen support for ARM).

Compile enlighten.c and grant_table.c from arch/arm.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 56b3f6d..b5d6ada 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -182,6 +182,16 @@
 
 source "mm/Kconfig"
 
+config XEN_DOM0
+	def_bool y
+	depends on XEN
+
+config XEN
+	bool "Xen guest support on ARM64 (EXPERIMENTAL)"
+	depends on ARM64 && OF
+	help
+	  Say Y if you want to run Linux in a Virtual Machine on Xen on ARM64.
+
 endmenu
 
 menu "Boot options"