ion: Add Prefetch IOCTL support for System Secure heap

Extend the IOCTL argument to support a vmid parameter for describing
which secure pool the prefetch is for. Also allow multiple different
memory sizes to be given to more accurately describe the future
allocations.

Change-Id: I8a91ef2f8ecf2075c18c963b4c82ca0e97ea770d
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>