Input: gpio_keys - add support for device-tree platform data

This patch enables fetching configuration data, which is normally provided
via platform_data, from the device-tree instead.

If the device is configured from device-tree data, the platform_data struct
is not used, and button data needs to be allocated dynamically. Big part of
this patch deals with confining pdata usage to the probe function, to make
this possible.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>