platform/x86: surfacepro3: Support for wakeup from suspend-to-idle

Modify surface_button_notify() to make it wake up the system from
suspend-to-idle (by reporting "hard" wakeup events while suspended)
and add wakeup initialization to surface_button_add() for wakeup
events reported by this driver to work at all.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=198389


Reported-by: Valentin Manea <valy@mrs.ro>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Valentin Manea <valy@mrs.ro>