Staging/iio/adc/touchscreen/MXS: add proper clock handling

The delay units inside the LRADC depend on the presence of a 2 kHz clock.
This change enables the clock to be able to use the delay unit for the
touchscreen part of the driver.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Tested-by: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>
Tested-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>