Input: rotary-encoder - update docs according to the latest API changes

The old device property API is about to be removed, so
explaing how to use complete software nodes instead.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://lore.kernel.org/r/20210304090948.27014-1-heikki.krogerus@linux.intel.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>