gpu: ion: Add explicit sync ioctl

This is deprecated in favor of using the dma_buf api which will
automatically sync a buffer to memory when it is mapped to a device.
However, that functionality is not ready, so this patch adds the
ability to sync a buffer explicitly.

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>