dt-bindings: adxl372: Document the adxl372 I2C bindings

The adxl372 is designed to communicate in either SPI or I2C protocol.
This patch adds the documentation of device tree bindings for adxl372
I2C.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>