ion: Add custom_compat_ioctl

Ion now has a compat_ioctl to deal with userspace compatibility but
there is currently no way to cleanly update custom ioctls. Add a
custom_compat_ioctl field to call when appropriate. It is up to the
client to decide when is appropriate to have this ioctl.

Change-Id: I3acbf7936cad2fa72df1b150cc0248598e22418c
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>