boards: st: stm32_f3_disco: add zephyr dtcm chosen property

In the STM32F3 series, the CCM RAM can interface with
the Arm Cortex-M4 core via the I-BUS, which makes it
possible to run the vector table relocation test.

See Refman 0316 Page 50.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>