dt-bindings: display: stm32: add supply property to DSI controller

This patch adds documentation of a new property phy-dsi-supply to the
STM32 DSI controller.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1557826556-10079-2-git-send-email-yannick.fertre@st.com