ARM: mx51_defconfig: change CONFIG_LEDS_CLASS 'm' to 'y'

This patch is to fix the warning below.

arch/arm/configs/mx51_defconfig:113:warning: symbol value 'm' invalid for LEDS_CLASS

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>