iio:adc:bcm_iproc: Drop of_match_ptr protection and switch to mod_devicetable.h

This driver cannot be instantiated from ACPI due to it's use of
syscon_regmap_lookup_by_phandle() but in the interests of clearing
this anti pattern out of IIO, let us switch to an explicit
check in Kconfig and remove the protections on the of_match_table
The switch of header is because we only use of_device_id
in here and that is defined in mod_devicetable.h not of.h.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Raveendra Padasalagi <raveendra.padasalagi@broadcom.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>