iio: imx7d_adc: Use devm_iio_device_register()

Use devm_iio_device_register() and drop imx7d_adc_remove().

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: Chris Healy <cphealy@gmail.com>
Cc: linux-iio@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>