regulator: twl: Remove references to 32kHz clock from DT bindings

Due to the lack of a generic clock API we'd had the 32kHz clock in the
regulator driver but this is definitely a Linux-specific thing and now
we have a clock API hopefully the code can be moved elsewhere. Try to
avoid getting DTs deployed relying on the 32kHz clock by removing it
from the bindings, grep seems to tell me it's not currently used anyway.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>