platform: x86: intel-vbtn: Wake up the system from suspend-to-idle

Allow the intel-vbtn driver to wake up the system from suspend-to-idle
by configuring its platform device as a wakeup one by default and
switching it over to a system wakeup events triggering mode during
system suspend transitions.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>