iio: backend: extend features

Extend backend features with new calls needed later on this
patchset from axi version of ad3552r.

The follwoing calls are added:

iio_backend_ddr_enable()
	enable ddr bus transfer
iio_backend_ddr_disable()
	disable ddr bus transfer
iio_backend_data_stream_enable()
	enable data stream over bus interface
iio_backend_data_stream_disable()
	disable data stream over bus interface
iio_backend_data_transfer_addr()
	define the target register address where the DAC sample
	will be written.

Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241028-wip-bl-ad3552r-axi-v0-iio-testing-v9-3-f6960b4f9719@kernel-space.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>