serial: mctrl_gpio: Add missing module license

As the mctrl_gpio driver can be built as a module, it needs to have its
license specified with MODULE_LICENSE. Otherwise, it cannot access
required symbols exported through EXPORT_SYMBOL_GPL.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>