platform/x86: asus-nb-wmi: Support left round button on N56VB

The N56VB laptop has a round button located on the left side above the
keyboard. Map it to F13 since it does not have any predeterminated
purpose.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>