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>