iio: adc: spear_adc: align headers with IWYU principle

iio: adc: spear_adc: align headers with IWYU principle

Remove unused includes and add what is being used:

#include <linux/array_size.h> // for ARRAY_SIZE
#include <linux/bits.h>  // for GENMASKxx
#include <linux/dev_printk.h> // for dev_err_probe, dev_info
#include <linux/math.h>  // for DIV_ROUND_UP
#include <linux/mutex.h> // for struct mutex
#include <linux/types.h> // for uXX definitions
#include <linux/iio/types.h> // for IIO_CHAN_INFO_*

Signed-off-by: Rodrigo Gobbi <rodrigo.gobbi.7@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>