ARM: allow memory hotplug/hotremove

Add ARM to the supported list of architectures for MEMORY_HOTPLUG.  For
ARM, the selection of MEMORY_HOTPLUG/REMOVE is specific to the sub-arch
and has to be explicitly enabled by the sub-arch.

Signed-off-by: Bryan Huntsman <bryanh@codeaurora.org>
(cherry picked from commit 45f580d9c36b93204882dffc6fb9f4a254c3d34a)