ion: system_heap: Add a new allocation method

Allocating new secure pages has very high latency. Ensure that the page
pool is completely empty before allocating new pages, splitting large order
pages as required.

Change-Id: I1353c5d9bdfd5c525c0c781c524630577c84b95e
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>