dt-bindings: display: stm32: remove st-display-subsystem parent node requirement

There is no need anymore to have a "st-display-subsystem" parent node
in the device tree for the ltdc.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-6-git-send-email-philippe.cornu@st.com