iio: st_sensors: add device tree bindings

This adds some basic, simple device tree bindings to the STMicro
MEMS sensor drivers.

Cc: devicetree@vger.kernel.org
Cc: Lee Jones <lee.jones@linaro.org>
Cc: Denis CIOCCA <denis.ciocca@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/devicetree/bindings/iio/st-sensors.txt

+STMicroelectronics MEMS sensors
+
+The STMicroelectronics sensor devices are pretty straight-forward I2C or
+SPI devices, all sharing the same device tree descriptions no matter what
+type of sensor it is.
+
+Required properties:
+- compatible: see the list of valid compatible strings below
+- reg: the I2C or SPI address the device will respond to
+
+Optional properties:
+- vdd-supply: an optional regulator that needs to be on to provide VDD
+  power to the sensor.
+- vddio-supply: an optional regulator that needs to be on to provide the
+  VDD IO power to the sensor.
+- st,drdy-int-pin: the pin on the package that will be used to signal
+  "data ready" (valid values: 1 or 2). This property is not configurable
+  on all sensors.
+
+Sensors may also have applicable pin control settings, those use the
+standard bindings from pinctrl/pinctrl-bindings.txt.
+
+Valid compatible strings:
+
+Accelerometers:
+- st,lsm303dlh-accel
+- st,lsm303dlhc-accel
+- st,lis3dh-accel
+- st,lsm330d-accel
+- st,lsm330dl-accel
+- st,lsm330dlc-accel
+- st,lis331dlh-accel
+- st,lsm303dl-accel
+- st,lsm303dlm-accel
+- st,lsm330-accel
+
+Gyroscopes:
+- st,l3g4200d-gyro
+- st,lsm330d-gyro
+- st,lsm330dl-gyro
+- st,lsm330dlc-gyro
+- st,l3gd20-gyro
+- st,l3g4is-gyro
+- st,lsm330-gyro
+
+Magnetometers:
+- st,lsm303dlhc-magn
+- st,lsm303dlm-magn
+- st,lis3mdl-magn
+
+Pressure sensors:
+- st,lps001wp-press
+- st,lps25h-press
+- st,lps331ap-press