iio: dac: ad5758: Add support for hard reset

The ad5758 has a hardware reset active low input pin. This patch adds a
devicetree entry for a reset GPIO and a new ad5758_reset() function.
During
initialization, it is checked if the reset property is specified and the
the GPIO is being asserted, therefore the device will become active.

When the reset function is called, if the gpio_reset var is set, then
the
GPIO will be toggled, otherwise a software reset is performed.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>