arm: board: 96b_stm32_sensor_mez: enable USART3

This patchset enables USART3 on the 96Boards STM32 Mezzanine.
It is broken out to J10 Grove Connector.

Changes:

- Enabled USART3 in board dts.
- Updated board index.rst with uart pinouts.
- soc dtsi: enabled usart3.

Test: Tested USART3 as console at 115200 baud

Signed-off-by: Sahaj Sarup <sahaj.sarup@linaro.org>