xen/arm64: Don't compile cpu hotplug

On ARM64, when CONFIG_XEN=y, the compilation will fail because CPU hotplug is
not yet supported with XEN. For now, disable it.
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>

drivers/xen/Makefile

-ifneq ($(CONFIG_ARM),y)
+ifeq ($(filter y, $(CONFIG_ARM) $(CONFIG_ARM64)),)
 obj-y	+= manage.o
 obj-$(CONFIG_HOTPLUG_CPU)		+= cpu_hotplug.o
 endif