ARM: dts: imx28-tx28: fix dtc warning

This fixes the following dtc warning by removing the unnecessary unit:

Warning (unit_address_vs_reg): Node /matrix-keypad@0 has a unit name,
but no reg property
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-By: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx28-tx28.dts

@@ -173,7 +173,7 @@ backlight {
 		default-brightness-level = <50>;
 	};
 
-	matrix_keypad: matrix-keypad@0 {
+	matrix_keypad: matrix-keypad {
 		compatible = "gpio-matrix-keypad";
 		col-gpios = <
 			&gpio5 0 GPIO_ACTIVE_HIGH