Input: sirfsoc-onkey - implement open and close methods

We can control whetehr device generates interrupts or not so let's
implement open and close methods of input device so that we do not do any
processing until there are users.

Tested-by: Xianglong Du <Xianglong.Du@csr.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>