Input: add support for polling to input devices

Separating "normal" and "polled" input devices was a mistake, as often we
want to allow the very same device work on both interrupt-driven and
polled mode, depending on the board on which the device is used.

This introduces new APIs:

- input_setup_polling
- input_set_poll_interval
- input_set_min_poll_interval
- input_set_max_poll_interval

These new APIs allow switching an input device into polled mode with sysfs
attributes matching drivers using input_polled_dev APIs that will be
eventually removed.

Tested-by: Michal Vokáč <michal.vokac@ysoft.com>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>