iio: adc: ad7768-1: reorganize driver headers

Remove kernel.h since it adds a lot of unnecessary dependencies.
Add specific headers to ensure all functions and macros used in the driver
are directly declared.

Suggested-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Santos <Jonathan.Santos@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Link: https://patch.msgid.link/1f7677d31a0165cb30d7eb3b4d613e1337937f9a.1746662899.git.Jonathan.Santos@analog.com


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