iio:light:cm3605: Switch to generic firmware properties.

This enables use of other firmware types with minimal driver changes.
Part of an ongoing effort to move all IIO drivers over to generic
accessors in order to reduce the chance of of_* versions being
copied into new drivers.  Also updated the headers to reflect this change
including using mod_devicetable.h for struct of_device_id definition
rather than going via of.h

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>