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>