rtc: rtc-mxc: adapt to the new i.mx clock framework

i.mx drivers should use clk_prepare_enable/clk_disable_unprepare() in
order to avoid clk warnings.

While at it, convert to devm_clk_get() since other devm_ functions are
used in this driver and it can also save some clk_put() calls.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>