This change allows specify interrupt for buttons separately form gpio,
potentially allowing to form several "clusters" of buttons on
different interrupts.

Button defined without both gpio and irq in device tree is a hared error
instead of a warning now.
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>