platform/x86: pcengines-apuv2: use KEY_RESTART for front button

The keycode KEY_RESTART is more appropriate for the front button,
as most people use it for things like restart or factory reset.

Signed-off-by: Enrico Weigelt <info@metux.net>
Fixes: f8eb0235 ("x86: pcengines apuv2 gpio/leds/keys platform driver")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>