ARM: dts: stm32: Pull UART4 RX high on AV96

There is no dedicated pull resistor on the AV96 UART4 (console UART)
pin. In case there is no UART adapter installed on the AV96, the line
is floating and can trigger reception of garbage characters, which in
turn can abort U-Boot autoboot. Add default pull up to mitigate this
problem.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>