coresight-dummy: add device tree property for source devices

Add device tree property to indicate the type of dummy device.
This can be used to distinguish between different types of dummy
devices.

Change-Id: Ic729c64c0acf8e38147ec2d709d51081fafd40f9
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>