iio: amplifiers: ad8366: replace reset-gpio with reset controller

iio: amplifiers: ad8366: replace reset-gpio with reset controller

Remove reset_gpio from the device state struct and use the reset_control
interface instead, using a local variable, as it is not being used
anywhere else. The reset controller init is moved out from the switch case
and optionally initialized for every device variant. Although not all
devices have a reset pin the code does not need to change if it is not
wired.

Signed-off-by: Rodrigo Alencar <rodrigo.alencar@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>