ion: Add dummy driver for testing

Provide a basic dummy driver to register the ion device
and to install basic SYSTEM and SYSTEM_CONTIG heaps.

This allows for basic testing with ION without having
access to drivers or systems that have been enabled to use
ION.

Cc: Colin Cross <ccross@android.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Jesse Barker <jesse.barker@arm.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>