Add stm32f746g_disco board to MCO sample.
This board has 2 MCO instances so useful to show multiple instances
as well as checking the implementation.
Signed-off-by: Joakim Andersson <joerchan@gmail.com>
Group all STM32 board samples in samples/boards/st to
keep samples/boards/ organized by vendor
Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>