rtc: mxc_v2: remove __exit annotation

The mxc_rtc_remove is incorrectly annotated as __exit:

`mxc_rtc_remove' referenced in section `.data' of drivers/rtc/rtc-mxc_v2.o: defined in discarded section `.exit.text' of drivers/rtc/rtc-mxc_v2.o

This should not be done, as devices can be dynamically bound
and unbound to a driver.

Fixes: 54c47014b474 ("rtc: add mxc driver for i.MX53 SRTC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>