drm/bridge/synopsys: dsi: add power on/off optional phy ops

Add power on & off optional physical operation functions, helpful to
program specific registers of the DSI physical part.

Signed-off-by: Yannick Fertré <yannick.fertre@st.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Tested-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1558952499-15418-2-git-send-email-yannick.fertre@st.com