Input: matrix-keymap - provide a proper module license

The matrix-keymap module is currently lacking a proper module license,
add one so we don't have this module tainting the entire kernel. This
issue has been present since commit 1932811f (Input: matrix-keymap -
uninline and prepare for device tree support)

Signed-off-by: Florian Fainelli <florian@openwrt.org>
CC: stable@vger.kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>