board: stm32mp1: display reference only for STMicroelectronics board

Display the reference MBxxxx found in OTP49
only for STMicroelectronics boards when CONFIG_CMD_STBOARD
is activated.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>

# Conflicts:
#	board/st/stm32mp1/stm32mp1.c