ARM: mxs_defconfig: Change CONFIG_RTC_CLASS 'm' to 'y'

The patch fixes the warning below.

arch/arm/configs/mxs_defconfig:92:warning: symbol value 'm' invalid for RTC_CLASS

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