[PATCH] I2C: add i2c sensor_device_attribute and macros

This patch creates a new header with a potential standard i2c sensor
attribute type (which simply includes an int representing the sensor
number/index) and the associated macros, SENSOR_DEVICE_ATTR to define
a static attribute and to_sensor_dev_attr to get a
sensor_device_attribute reference from an embedded device_attribute
reference.

Signed-off-by: Yani Ioannou <yani.ioannou@gmail.com>