mfd: ti_am335x_tscadc: Keep ADC interface on if child is wakeup capable

If a child device like touchscreen is wakeup capable, then keep ADC
interface on, so that a touching resistive screen will generate wakeup
event to the system.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>