The only possible assignment of a function to get a voltage to
convert to a resistance is to use the internal function
ntc_adc_iio_read() which is only available when using IIO
and OF.

Bite the bullet and mandate OF and IIO, drop the read_uv()
callback abstraction and some ifdefs.

As no board is using the platform data, all users are using
OF and IIO anyway.

Cc: Peter Rosin <peda@axentia.se>
Cc: Chris Lesiak <chris.lesiak@licor.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20211125020841.3616359-4-linus.walleij@linaro.orgSigned-off-by: Guenter Roeck <linux@roeck-us.net>