arm64: Make section size configurable for memory hotplug

Currently On arm64, memory section size is hard-coded to 1G.
Make this configurable if memory-hotplug is enabled, to support
more finer granularity for hotplug-able memory.

Change-Id: Id889f93537df8ae7e72094b552cff735d31048e6
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>