IIO: add si7020 driver

IIO: add si7020 driver

This patch adds support to the Industrial IO subsystem
for the Silicon Labs Si7013/20/21 Relative Humidity and
Temperature Sensors.

Website: http://www.silabs.com/products/sensors/humidity-sensors/Pages/si7013-20-21.aspx

These are i2c devices which measure relative humidity
and temperature and all use the same protocol. The
Si7013 has an additional input with programmable
linearization which is not supported because that's
complicated and I didn't need it.

Signed-off-by: David Barksdale <dbarksdale@uplogix.com>
--
Changes since v1:
* Renamed to si7020 and replaced Si701x/2x with Si7013/20/21.
* Removed unneeded mutex.
* Pre-computed floating-point constant expressions.
* Removed address_list and I2C_CLASS_HWMON.

Changes since v2:
* Return correct raw sensor values.
* Rename dev variable to indio_dev.
* Issue a software reset command during probe.
* Un-broke string literal.

Changes since v3:
* enum changed to #define
Signed-off-by: Jonathan Cameron <jic23@kernel.org>