ARM: mx5/mx53_loco: Fix build warning related to gpio_keys_button structure

Fix the following warning:

CC      arch/arm/mach-mx5/board-mx53_loco.o
arch/arm/mach-mx5/board-mx53_loco.c:203: warning: initialization discards qualifiers from pointer target type

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>