boards: stm32h747i_disco: Assign usart8 to m4 target

Resource sharing between m7 and m4 core was a bit abusive
as the whole SERIAL ports where disabled on m4 core.
Since it is possible to use a finer grain resource sharing and
having no serial node configured could lead to compilation issue
in some samples assign usart8 to m4 target.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>