iio:potentiometer:ad5272: Drop of_match_ptr and CONFIG_OF protections.

These prevent use of this driver with ACPI via PRP0001 and are
an example of an anti pattern I'm trying to remove from IIO.
Drop them to remove this restriction.

Also added mod_devicetable.h include given use of struct of_device_id
which is defined in that header.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Phil Reid <preid@eletromag.com.au>
Link: https://lore.kernel.org/r/20200910173242.621168-2-jic23@kernel.org