ion: Support an option to allocate buffers in 4KB chunks

For performance profiling, we want to create worst case
scenarios where system is quite fragmented and cannot
allocate buffers in chunks more than 4KB. Enable an
option which forcibly allocates in 4KB chunks only.

Change-Id: I323603de4c0dc04cdf247bf4923b94b8ff4cd9f0
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>