gpu: ion: Add support for specifying a default prefetch size

Some heaps now support prefetching memory before allocation occurs.
This currently requires a size to be given to prefetch a given size.
Clients may not know what size they need to prefetch for a given
heap. Since the optimal size may change with the target, add an option
into devicetree to specify what size should be prefetched by default.
Clients may specify a size of 0 to prefetch a the default size.

Change-Id: If15e4051d53fe284129c309101c41ee423f890f9
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>