iio: imu: adis: Add custom ops struct

iio: imu: adis: Add custom ops struct

This patch introduces a custom ops struct letting users define custom read and
write functions. Some adis devices might define a completely different spi
protocol from the one used in the default implementation.

Co-developed-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Signed-off-by: Ramona Gradinariu <ramona.gradinariu@analog.com>
Co-developed-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Co-developed-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Signed-off-by: Robert Budai <robert.budai@analog.com>
Link: https://patch.msgid.link/20250217105753.605465-2-robert.budai@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>