ARM: dts: stm32: move ltdc pinctrl on stm32mp157a dk1 board

The ltdc pinctrl must be in the display controller node and
not in the peripheral node (hdmi bridge).

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>