mfd: remove DS1WM clock handling

This driver requests a clock that usually is supplied by the MFD in which
the DS1WM is contained. Currently, it is impossible for a MFD to register
their clocks with the generic clock API due to different implementations
across architectures.
For now, this patch removes the clock handling from DS1WM altogether,
trusting that the MFD enable/disable functions will switch the clock if
needed. The clock rate is obtained from a new parameter in driver_data.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>