rtc: mxc: use a second rtc clock

The mxc RTC needs two clocks, one for the input
reference, and one for the IP. But this driver
was only using one clock (for the reference).
This patch add the second clock (for the IP).

Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>