iio:adc:mcp3911: Switch to generic firmware properties.

This allows use of the driver with other types of firmware such as ACPI
PRP0001 based probing.

Also part of a general attempt to remove direct use of of_ specific
accessors from IIO.

Added an include for mod_devicetable.h whilst here to cover the
struct of_device_id definition.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Kent Gustavsson <kent@minoris.se>
Reviewed-by: Marcus Folkesson <marcus.folkesson@gmail.com>