hwmon/ltc2990: Generalise DT to fwnode support

hwmon/ltc2990: Generalise DT to fwnode support

ltc2990 will now use device_property_read_u32_array() instead of
of_property_read_u32_array() - allowing the use of software nodes
via fwnode_create_software_node().

This allows code using i2c_new_device() to specify a default
measurement mode for the LTC2990 via fwnode_create_software_node().

Signed-off-by: Max Staudt <max@enpas.org>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20190819121618.16557-2-max@enpas.org
Signed-off-by: Guenter Roeck <linux@roeck-us.net>