Add binding information for DA9061 onkey.

This patch updates the compatible string "dlg,da9061-onkey" to support
DA9061, removes the reference to KEY_SLEEP (which the driver no longer
supports) and fixes a typo in the example for DA9063.

Supporting KEY_SLEEP was not the general convention and the typical
solution should have been for KEY_POWER to support both cases of suspend
and S/W power off. This change was sent to the DA9063 ONKEY device
driver in a separate patch, but the documentation was not updated at
that time.
- f889beaa Report KEY_POWER instead of KEY_SLEEP during power key-press

This patch also adds two new examples, one for DA9062 and one for DA9061.
The DA9061 examples uses a fall-back compatible string for the DA9062
onkey driver.
Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>