boards: arm: stm32h747i_disco: rework uart split between CM4 and CM7

Move uart/usart pinctrl to common file.
Set status in dedicated m7 or m4 file.
But only one core can control a peripheral instance.
Arbitrary default choice:
usart1 --> m7
uart8 --> m4

Signed-off-by: Alexandre Bourdiol <alexandre.bourdiol@st.com>
3 files changed