ARM: dts: pxa: fix the rtc controller

The RTC controller is fed by an external fixed 32kHz clock. Yet the
driver wants to acquire this clock, even though it doesn't make any use
of it, ie. doesn't get the rate to make calculation.

Therefore, use the exported 32.768kHz clock in the PXA clock tree to
make the driver happy and working.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>