ion: add test device for unit tests to interact with dma_bufs

Add a /dev/ion-test device that will be created if CONFIG_ION_TEST
is set.  The device accepts a dma_buf fd and allows reading and
writing to the backing memory using DMA-like apis or kernel mapping
apis.  Can be used to test the dma_buf mapping ops, including
the ion implementations, from userspace.

Change-Id: I30703ba69cd75bdfe7767ac642e5f0cacd8d0478
Signed-off-by: Colin Cross <ccross@android.com>