ARM: DT: STiH407: Add DRM dt nodes

This patch adds the DRM/KMS dt nodes.
This node can't be in stih407-family.dtsi file because in the future we
will integrate a new stih418-b2199 board. It's a stih407 family board
with different drm/kms dt nodes.
That is why i created the stih407.dtsi file.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>