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>

arch/arm64/boot/dts/rockchip/rk3399-gru-kevin.dts

@@ -249,6 +249,10 @@ trackpad@4a {
 		pinctrl-0 = <&trackpad_int_l>;
 		interrupt-parent = <&gpio1>;
 		interrupts = <4 IRQ_TYPE_LEVEL_LOW>;
+		linux,gpio-keymap = <KEY_RESERVED
+				     KEY_RESERVED
+				     KEY_RESERVED
+				     BTN_LEFT>;
 		wakeup-source;
 	};
 };