boards: arm: Add support for SensorTile.box board

The SensorTile.box is a board designed for IoT applications
embedding a wide range of intelligent low power MEMS sensors,
a STM32L4 microcontroller to manage sensor configuration and
process sensor output data, a micro-USB battery charging
interface and an ST Bluetooth Low Energy module for wireless
communication with a BLE-enabled smartphone.

Signed-off-by: Armando Visconti <armando.visconti@st.com>