imx31: fix parent clock for rtc

According to imx31 reference manual the signal from external low
frequency clock is sent to RTC clock.
The patch makes redundant the previously defined mxc_rtc clock.

Signed-off-by: Vladimir Zapolskiy <vzapolskiy@gmail.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>