mm/Kconfig: Enable MEMORY_HOTPLUG for arm64

Add arm64 to the list of architectures which supports
memory hotplug.

Change-Id: Iefeb8294bf06eaebb17a3b3aa8b33bb3b7133099
Signed-off-by: Arun KS <arunks@codeaurora.org>
Signed-off-by: Srivatsa Vaddagiri <vatsa@codeaurora.org>
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>