Arm backend: Set default allocation pool to 20MB

This is making memory fit on more targets. The intention is for this
is to be easier to change from the run.sh flow and then this value
will probably be bumped back to the bigger value out of the box again
to make large model testing more convenient.

Signed-off-by: Zingo Andersen <zingo.andersen@arm.com>
Change-Id: Id0e5f0d453bddc8e8fb0db65a4fd72bc7c1671ea