Input: input-polldev - add sysfs interface for controlling poll interval

Sysfs entry for reading and setting of the polling interval. If the
interval is set to 0, polling is stopped. Polling is restarted when
interval is changed to non-zero.

sysfs entries:
poll = current polling interval in msec (RW)
max = max allowed polling interval (RO)
min = min allowed polling interval (RO)

Minimum and maximum limit for interval can be set while setting up the
device.

Interval can be adjusted even if the input device is not currently open.

[dtor@mail.ru: add kernel doc markup for the new fields]
Signed-off-by: Samu Onkalo <samu.p.onkalo@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>