iio: light: cm32181: Change reg_init to use a bitmap of which registers to init

This is a preparation patch for reading some ACPI tables which give
init values for multiple registers.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>