arm64: Make section size configurable for hotplug

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

Change-Id: I5327ead37069176f4ba34657133fab303fb15b82
Signed-off-by: Arun KS <arunks@codeaurora.org>