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>

arch/arm/configs/mxs_defconfig

@@ -89,7 +89,7 @@ CONFIG_DISPLAY_SUPPORT=m
 # CONFIG_USB_SUPPORT is not set
 CONFIG_MMC=y
 CONFIG_MMC_MXS=y
-CONFIG_RTC_CLASS=m
+CONFIG_RTC_CLASS=y
 CONFIG_RTC_DRV_DS1307=m
 CONFIG_DMADEVICES=y
 CONFIG_MXS_DMA=y