ion: Kconfig: Provide a support to allocate 4KB chunks

For performance analysis, we may want to create a scenario
where buffers from ION SYSTEM heap can get allocated in 4KB
chunks. To mimic that situation forcibly, provide an option
which can force allocations to be in 4KB chunks.

Change-Id: Id67e41669efa60dbf59b085177808a3ae8f333d7
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>