Allow the driver to work with device tree support.

Based on initial patch submission from Peter Fox.

Tested on a imx7d-sdb board connected to a SHT15 board via Mikro Bus.
Signed-off-by: NMarco Franchi <marco.franchi@nxp.com>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>