iio: adc: aspeed: Deassert reset in probe

The ASPEED SoC must deassert a reset in order to use the ADC peripheral.

The device tree bindings are updated to document the resets phandle, and
the example is updated to match what is expected for both the reset and
clock phandle. Note that the bindings should have always had the reset
controller, as the hardware is unusable without it.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>