ARM: dts: stm32: Repair I2C2 operation on AV96

The I2C2 uses different pinmux on AV96, use correct pinmux and
also add comments about the I2C being present on the "low-speed"
expansion connector X6.

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>