iio: adc: ad7124: Silence no spi_device_id warnings

iio: adc: ad7124: Silence no spi_device_id warnings

SPI devices use the spi_device_id for module autoloading even on
systems using device tree, after commit 5fa6863ba692 ("spi: Check
we have a spi_device_id for each DT compatible"), kernel warns as
follows since the spi_device_id is missing:

SPI driver ad7124 has no spi_device_id for adi,ad7124-4
SPI driver ad7124 has no spi_device_id for adi,ad7124-8

Add spi_device_id entries to silence the warnings, and ensure driver
module autoloading works.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20220921163620.805879-6-weiyongjun@huaweicloud.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>