arm64: dts: rockchip: enable touchpad button for rk3399-gru-kevin

Adding the linux,gpio-keymap entry also has
the side-effect of making the driver register
the touchpad as a touchpad rather than another
touchscreen.

The index for BTN_LEFT was found by trial and error.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>