Input: atmel_mxt_ts - support regulator supplies

Input: atmel_mxt_ts - support regulator supplies

This adds the code for the Atmel touchscreens such as mXT224 to obtain
power regulators for the supply voltages AVDD and VDD. On mobile phones
such as Samsung GT-I8190 (Golden) this is needed to explicitly bring power
online.

We just enable the regulators at probe() and disable them at remove()
or in the error path for now.

As regulators are naturally stubbed if not available, this should have no
impact on existing systems.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201104153032.1387747-3-linus.walleij@linaro.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>