iio: dummy: Add virtual registers for dummy device

We need a way to store events generated by iio_dummy_evgen module,
in order to correctly process IRQs in iio_simple_dummy_events.

For the moment, we add two registers:

* id_reg  - ID register, stores the source of the event
* id_data - DATA register, stores the type of the event

e.g echo 4 > /sys/bus/iio/devices/iio_evgen/poke2

id_reg 0x02, id_data 0x04

This means, event of type 4 was generated by fake device 2.

We currently use a hardcoded mapping of virtual events to IIO events.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>