staging:iio:dac:ad5064: Convert to extended channel info attributes

Use extended channel info attributes for the powerdown, powerdown_mode and
powerdown_mode_available attributes.

Note that this patch moves the chip info defintion around to avoid having to use
forward declarations for the extended channel info attributes callbacks.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>