mfd: twl6040: Optional clk32k clock handling

In certain boards the source for the clk32k clock can be gated. In these
boards the clk32k clock can be provided to the driver and it is going to be
enabled/disabled when it is needed.
If the clk32k clock is not provided the driver will assume that it is always
running.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>