rtc: Clean drivers/rtc/Makefile

drivers/rtc/Makefile

drivers/rtc/bfin_rtc.c

drivers/rtc/ds12887.c

drivers/rtc/ds1302.c

drivers/rtc/ds1306.c

drivers/rtc/ds1307.c

drivers/rtc/ds1337.c

drivers/rtc/ds1374.c

drivers/rtc/ds1556.c

drivers/rtc/ds164x.c

drivers/rtc/ds174x.c

drivers/rtc/ds3231.c

drivers/rtc/m41t11.c

drivers/rtc/m41t60.c

drivers/rtc/m48t35ax.c

drivers/rtc/max6900.c

drivers/rtc/mc146818.c

drivers/rtc/mcfrtc.c

drivers/rtc/mk48t59.c

drivers/rtc/mpc5xxx.c